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Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

[PATCH 00/14] PV ticket locks without expanding spinlock

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Hi all,

This is a revised version of the pvticket lock series.

The early part of the series is mostly unchanged: it converts the bulk
of the ticket lock code into C and makes the "small" and
"large"
ticket code common.  The only changes are the incorporation of various
review comments.

The latter part of the series converts from pv spinlocks to pv ticket
locks (ie, using the ticket lock fastpath as-is, but adding pv ops for
the ticketlock slowpaths).

The significant difference here is that rather than adding a new
ticket_t-sized element to arch_spinlock_t - effectively doubling the
size - I steal the LSB of the tickets themselves to store a bit.  This
allows the structure to remain the same size, but at the cost of
halving the max number of CPUs (127 for a 8-bit ticket, and a hard max
of 32767 overall).

The extra bit (well, two, but one is unused) in indicates whether the
lock has gone into "slowpath state", which means one of its lockers
has entered its slowpath and has blocked in the hypervisor.  This
means the current lock-holder needs to make sure it gets kicked out of
the hypervisor on unlock.

The spinlock remains in slowpath state until the last unlock happens
(ie there are no more queued lockers).

This code survives for a while with moderate testing, (make -j 100 on
8 VCPUs on a 4 PCPU system), but locks up after about 20 iterations,
so there's still some race/deadlock in there (probably something
misordered), but I think the basic approach is sound.

Thanks,
	J

Jeremy Fitzhardinge (14):
  x86/ticketlock: clean up types and accessors
  x86/ticketlock: convert spin loop to C
  x86/ticketlock: Use C for __ticket_spin_unlock
  x86/ticketlock: make large and small ticket versions of spin_lock the
    same
  x86/ticketlock: make __ticket_spin_lock common
  x86/ticketlock: make __ticket_spin_trylock common
  x86/spinlocks: replace pv spinlocks with pv ticketlocks
  x86/ticketlock: collapse a layer of functions
  xen/pvticketlock: Xen implementation for PV ticket locks
  x86/pvticketlock: use callee-save for lock_spinning
  x86/ticketlock: don't inline _spin_unlock when using paravirt
    spinlocks
  x86/ticketlocks: when paravirtualizing ticket locks, increment by 2
  x86/ticketlock: add slowpath logic
  x86/ticketlocks: tidy up __ticket_unlock_kick()

 arch/x86/Kconfig                      |    3 +
 arch/x86/include/asm/paravirt.h       |   30 +---
 arch/x86/include/asm/paravirt_types.h |    8 +-
 arch/x86/include/asm/spinlock.h       |  236 +++++++++++++---------------
 arch/x86/include/asm/spinlock_types.h |   32 ++++-
 arch/x86/kernel/paravirt-spinlocks.c  |   52 +++++--
 arch/x86/xen/spinlock.c               |  281 +++++----------------------------
 kernel/Kconfig.locks                  |    2 +-
 8 files changed, 231 insertions(+), 413 deletions(-)

-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 01/14] x86/ticketlock: clean up types and accessors

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

A few cleanups to the way spinlocks are defined and accessed:
 - define __ticket_t which is the size of a spinlock ticket (ie, enough
   bits to hold all the cpus)
 - Define struct arch_spinlock as a union containing plain slock and
   the head and tail tickets
 - Use head and tail to implement some of the spinlock predicates.
 - Make all ticket variables unsigned.
 - Use TICKET_SHIFT to form constants

Most of this will be used in later patches.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h       |   24 ++++++++++--------------
 arch/x86/include/asm/spinlock_types.h |   20 ++++++++++++++++++--
 2 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 3089f70..d6d5784 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -56,11 +56,9 @@
  * much between them in performance though, especially as locks are out of
line.
  */
 #if (NR_CPUS < 256)
-#define TICKET_SHIFT 8
-
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
-	short inc = 0x0100;
+	unsigned short inc = 1 << TICKET_SHIFT;
 
 	asm volatile (
 		LOCK_PREFIX "xaddw %w0, %1\n"
@@ -79,7 +77,7 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t
*lock)
 
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
 {
-	int tmp, new;
+	unsigned int tmp, new;
 
 	asm volatile("movzwl %2, %0\n\t"
 		     "cmpb %h0,%b0\n\t"
@@ -104,12 +102,10 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
 		     : "memory", "cc");
 }
 #else
-#define TICKET_SHIFT 16
-
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
-	int inc = 0x00010000;
-	int tmp;
+	unsigned inc = 1 << TICKET_SHIFT;
+	unsigned tmp;
 
 	asm volatile(LOCK_PREFIX "xaddl %0, %1\n"
 		     "movzwl %w0, %2\n\t"
@@ -129,8 +125,8 @@ static __always_inline void
__ticket_spin_lock(arch_spinlock_t *lock)
 
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
 {
-	int tmp;
-	int new;
+	unsigned tmp;
+	unsigned new;
 
 	asm volatile("movl %2,%0\n\t"
 		     "movl %0,%1\n\t"
@@ -160,16 +156,16 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
 
 static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
 {
-	int tmp = ACCESS_ONCE(lock->slock);
+	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
-	return !!(((tmp >> TICKET_SHIFT) ^ tmp) & ((1 << TICKET_SHIFT)
- 1));
+	return !!(tmp.tail ^ tmp.head);
 }
 
 static inline int __ticket_spin_is_contended(arch_spinlock_t *lock)
 {
-	int tmp = ACCESS_ONCE(lock->slock);
+	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
-	return (((tmp >> TICKET_SHIFT) - tmp) & ((1 << TICKET_SHIFT) -
1)) > 1;
+	return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
 }
 
 #ifndef CONFIG_PARAVIRT_SPINLOCKS
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index dcb48b2..e3ad1e3 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -5,11 +5,27 @@
 # error "please don't include this file directly"
 #endif
 
+#include <linux/types.h>
+
+#if (CONFIG_NR_CPUS < 256)
+typedef u8  __ticket_t;
+#else
+typedef u16 __ticket_t;
+#endif
+
+#define TICKET_SHIFT	(sizeof(__ticket_t) * 8)
+#define TICKET_MASK	((__ticket_t)((1 << TICKET_SHIFT) - 1))
+
 typedef struct arch_spinlock {
-	unsigned int slock;
+	union {
+		unsigned int slock;
+		struct __raw_tickets {
+			__ticket_t head, tail;
+		} tickets;
+	};
 } arch_spinlock_t;
 
-#define __ARCH_SPIN_LOCK_UNLOCKED	{ 0 }
+#define __ARCH_SPIN_LOCK_UNLOCKED	{ { .slock = 0 } }
 
 typedef struct {
 	unsigned int lock;
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 02/14] x86/ticketlock: convert spin loop to C

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

The inner loop of __ticket_spin_lock isn't doing anything very special,
so reimplement it in C.

For the 8 bit ticket lock variant, we use a register union to get direct
access to the lower and upper bytes in the tickets, but unfortunately gcc
won't generate a direct comparison between the two halves of the register,
so the generated asm isn't quite as pretty as the hand-coded version.
However benchmarking shows that this is actually a small improvement in
runtime performance on some benchmarks, and never a slowdown.

We also need to make sure there's a barrier at the end of the lock loop
to make sure that the compiler doesn't move any instructions from within
the locked region into the region where we don't yet own the lock.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h |   58 +++++++++++++++++++-------------------
 1 files changed, 29 insertions(+), 29 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index d6d5784..f48a6e3 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -58,21 +58,21 @@
 #if (NR_CPUS < 256)
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
-	unsigned short inc = 1 << TICKET_SHIFT;
-
-	asm volatile (
-		LOCK_PREFIX "xaddw %w0, %1\n"
-		"1:\t"
-		"cmpb %h0, %b0\n\t"
-		"je 2f\n\t"
-		"rep ; nop\n\t"
-		"movb %1, %b0\n\t"
-		/* don't need lfence here, because loads are in-order */
-		"jmp 1b\n"
-		"2:"
-		: "+Q" (inc), "+m" (lock->slock)
-		:
-		: "memory", "cc");
+	register union {
+		struct __raw_tickets tickets;
+		unsigned short slock;
+	} inc = { .slock = 1 << TICKET_SHIFT };
+
+	asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
+		      : "+Q" (inc), "+m" (lock->slock) : :
"memory", "cc");
+
+	for (;;) {
+		if (inc.tickets.head == inc.tickets.tail)
+			goto out;
+		cpu_relax();
+		inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
+	}
+out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
@@ -105,22 +105,22 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
 	unsigned inc = 1 << TICKET_SHIFT;
-	unsigned tmp;
+	__ticket_t tmp;
 
-	asm volatile(LOCK_PREFIX "xaddl %0, %1\n"
-		     "movzwl %w0, %2\n\t"
-		     "shrl $16, %0\n\t"
-		     "1:\t"
-		     "cmpl %0, %2\n\t"
-		     "je 2f\n\t"
-		     "rep ; nop\n\t"
-		     "movzwl %1, %2\n\t"
-		     /* don't need lfence here, because loads are in-order */
-		     "jmp 1b\n"
-		     "2:"
-		     : "+r" (inc), "+m" (lock->slock),
"=&r" (tmp)
-		     :
-		     : "memory", "cc");
+	asm volatile(LOCK_PREFIX "xaddl %0, %1\n\t"
+		     : "+r" (inc), "+m" (lock->slock)
+		     : : "memory", "cc");
+
+	tmp = inc;
+	inc >>= TICKET_SHIFT;
+
+	for (;;) {
+		if ((__ticket_t)inc == tmp)
+			goto out;
+		cpu_relax();
+		tmp = ACCESS_ONCE(lock->tickets.head);
+	}
+out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 03/14] x86/ticketlock: Use C for __ticket_spin_unlock

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

If we don't need to use a locked inc for unlock, then implement it in C.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h |   32 +++++++++++++++++---------------
 1 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index f48a6e3..0170ba9 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -33,9 +33,21 @@
  * On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock
  * (PPro errata 66, 92)
  */
-# define UNLOCK_LOCK_PREFIX LOCK_PREFIX
+static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
+{
+	if (sizeof(lock->tickets.head) == sizeof(u8))
+		asm (LOCK_PREFIX "incb %0"
+		     : "+m" (lock->tickets.head) : : "memory");
+	else
+		asm (LOCK_PREFIX "incw %0"
+		     : "+m" (lock->tickets.head) : : "memory");
+
+}
 #else
-# define UNLOCK_LOCK_PREFIX
+static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
+{
+	lock->tickets.head++;
+}
 #endif
 
 /*
@@ -93,14 +105,6 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 
 	return tmp;
 }
-
-static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
-{
-	asm volatile(UNLOCK_LOCK_PREFIX "incb %0"
-		     : "+m" (lock->slock)
-		     :
-		     : "memory", "cc");
-}
 #else
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
@@ -144,15 +148,13 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 
 	return tmp;
 }
+#endif
 
 static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
 {
-	asm volatile(UNLOCK_LOCK_PREFIX "incw %0"
-		     : "+m" (lock->slock)
-		     :
-		     : "memory", "cc");
+	__ticket_unlock_release(lock);
+	barrier();		/* prevent reordering into locked region */
 }
-#endif
 
 static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
 {
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 04/14] x86/ticketlock: make large and small ticket versions of spin_lock the same

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Make the bulk of __ticket_spin_lock look identical for large and small
number of cpus.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h |   23 ++++++++---------------
 1 files changed, 8 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 0170ba9..1b81809 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -70,19 +70,16 @@ static __always_inline void __ticket_unlock_release(struct
arch_spinlock *lock)
 #if (NR_CPUS < 256)
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
-	register union {
-		struct __raw_tickets tickets;
-		unsigned short slock;
-	} inc = { .slock = 1 << TICKET_SHIFT };
+	register struct __raw_tickets inc = { .tail = 1 };
 
 	asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
-		      : "+Q" (inc), "+m" (lock->slock) : :
"memory", "cc");
+		      : "+r" (inc), "+m" (lock->tickets) : :
"memory", "cc");
 
 	for (;;) {
-		if (inc.tickets.head == inc.tickets.tail)
+		if (inc.head == inc.tail)
 			goto out;
 		cpu_relax();
-		inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
+		inc.head = ACCESS_ONCE(lock->tickets.head);
 	}
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
@@ -108,21 +105,17 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 #else
 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
 {
-	unsigned inc = 1 << TICKET_SHIFT;
-	__ticket_t tmp;
+	register struct __raw_tickets inc = { .tail = 1 };
 
 	asm volatile(LOCK_PREFIX "xaddl %0, %1\n\t"
-		     : "+r" (inc), "+m" (lock->slock)
+		     : "+r" (inc), "+m" (lock->tickets)
 		     : : "memory", "cc");
 
-	tmp = inc;
-	inc >>= TICKET_SHIFT;
-
 	for (;;) {
-		if ((__ticket_t)inc == tmp)
+		if (inc.head == inc.tail)
 			goto out;
 		cpu_relax();
-		tmp = ACCESS_ONCE(lock->tickets.head);
+		inc.head = ACCESS_ONCE(lock->tickets.head);
 	}
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 05/14] x86/ticketlock: make __ticket_spin_lock common

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Aside from the particular form of the xadd instruction, they're identical.
So factor out the xadd and use common code for the rest.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h |   42 ++++++++++++++++++--------------------
 1 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 1b81809..f722f96 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -67,13 +67,27 @@ static __always_inline void __ticket_unlock_release(struct
arch_spinlock *lock)
  * save some instructions and make the code more elegant. There really
isn't
  * much between them in performance though, especially as locks are out of
line.
  */
-#if (NR_CPUS < 256)
-static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
+static __always_inline struct __raw_tickets __ticket_spin_claim(struct
arch_spinlock *lock)
 {
-	register struct __raw_tickets inc = { .tail = 1 };
+	register struct __raw_tickets tickets = { .tail = 1 };
+
+	if (sizeof(lock->tickets.head) == sizeof(u8))
+		asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
+			      : "+r" (tickets), "+m" (lock->tickets)
+			      : : "memory", "cc");
+	else
+		asm volatile (LOCK_PREFIX "xaddl %0, %1\n"
+			     : "+r" (tickets), "+m" (lock->tickets)
+			     : : "memory", "cc");
 
-	asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
-		      : "+r" (inc), "+m" (lock->tickets) : :
"memory", "cc");
+	return tickets;
+}
+
+static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
+{
+	register struct __raw_tickets inc;
+
+	inc = __ticket_spin_claim(lock);
 
 	for (;;) {
 		if (inc.head == inc.tail)
@@ -84,6 +98,7 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t
*lock)
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
+#if (NR_CPUS < 256)
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
 {
 	unsigned int tmp, new;
@@ -103,23 +118,6 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 	return tmp;
 }
 #else
-static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
-{
-	register struct __raw_tickets inc = { .tail = 1 };
-
-	asm volatile(LOCK_PREFIX "xaddl %0, %1\n\t"
-		     : "+r" (inc), "+m" (lock->tickets)
-		     : : "memory", "cc");
-
-	for (;;) {
-		if (inc.head == inc.tail)
-			goto out;
-		cpu_relax();
-		inc.head = ACCESS_ONCE(lock->tickets.head);
-	}
-out:	barrier();		/* make sure nothing creeps before the lock is taken */
-}
-
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
 {
 	unsigned tmp;
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 06/14] x86/ticketlock: make __ticket_spin_trylock common

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Make trylock code common regardless of ticket size.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h       |   49 +++++++--------------------------
 arch/x86/include/asm/spinlock_types.h |    6 +++-
 2 files changed, 14 insertions(+), 41 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index f722f96..3afb1a7 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -98,48 +98,19 @@ static __always_inline void __ticket_spin_lock(struct
arch_spinlock *lock)
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
-#if (NR_CPUS < 256)
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
 {
-	unsigned int tmp, new;
-
-	asm volatile("movzwl %2, %0\n\t"
-		     "cmpb %h0,%b0\n\t"
-		     "leal 0x100(%" REG_PTR_MODE "0), %1\n\t"
-		     "jne 1f\n\t"
-		     LOCK_PREFIX "cmpxchgw %w1,%2\n\t"
-		     "1:"
-		     "sete %b1\n\t"
-		     "movzbl %b1,%0\n\t"
-		     : "=&a" (tmp), "=&q" (new), "+m"
(lock->slock)
-		     :
-		     : "memory", "cc");
-
-	return tmp;
-}
-#else
-static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
-{
-	unsigned tmp;
-	unsigned new;
-
-	asm volatile("movl %2,%0\n\t"
-		     "movl %0,%1\n\t"
-		     "roll $16, %0\n\t"
-		     "cmpl %0,%1\n\t"
-		     "leal 0x00010000(%" REG_PTR_MODE "0), %1\n\t"
-		     "jne 1f\n\t"
-		     LOCK_PREFIX "cmpxchgl %1,%2\n\t"
-		     "1:"
-		     "sete %b1\n\t"
-		     "movzbl %b1,%0\n\t"
-		     : "=&a" (tmp), "=&q" (new), "+m"
(lock->slock)
-		     :
-		     : "memory", "cc");
-
-	return tmp;
+	arch_spinlock_t old, new;
+
+	old.tickets = ACCESS_ONCE(lock->tickets);
+	if (old.tickets.head != old.tickets.tail)
+		return 0;
+
+	new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
+
+	/* cmpxchg is a full barrier, so nothing can move before it */
+	return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) ==
old.head_tail;
 }
-#endif
 
 static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
 {
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index e3ad1e3..72e154e 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -9,8 +9,10 @@
 
 #if (CONFIG_NR_CPUS < 256)
 typedef u8  __ticket_t;
+typedef u16 __ticketpair_t;
 #else
 typedef u16 __ticket_t;
+typedef u32 __ticketpair_t;
 #endif
 
 #define TICKET_SHIFT	(sizeof(__ticket_t) * 8)
@@ -18,14 +20,14 @@ typedef u16 __ticket_t;
 
 typedef struct arch_spinlock {
 	union {
-		unsigned int slock;
+		__ticketpair_t head_tail;
 		struct __raw_tickets {
 			__ticket_t head, tail;
 		} tickets;
 	};
 } arch_spinlock_t;
 
-#define __ARCH_SPIN_LOCK_UNLOCKED	{ { .slock = 0 } }
+#define __ARCH_SPIN_LOCK_UNLOCKED	{ { 0 } }
 
 typedef struct {
 	unsigned int lock;
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 07/14] x86/spinlocks: replace pv spinlocks with pv ticketlocks

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Rather than outright replacing the entire spinlock implementation in
order to paravirtualize it, keep the ticket lock implementation but add
a couple of pvops hooks on the slow patch (long spin on lock, unlocking
a contended lock).

Ticket locks have a number of nice properties, but they also have some
surprising behaviours in virtual environments.  They enforce a strict
FIFO ordering on cpus trying to take a lock; however, if the hypervisor
scheduler does not schedule the cpus in the correct order, the system can
waste a huge amount of time spinning until the next cpu can take the lock.

(See Thomas Friebel's talk "Prevent Guests from Spinning Around"
http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.)

To address this, we add two hooks:
 - __ticket_spin_lock which is called after the cpu has been
   spinning on the lock for a significant number of iterations but has
   failed to take the lock (presumably because the cpu holding the lock
   has been descheduled).  The lock_spinning pvop is expected to block
   the cpu until it has been kicked by the current lock holder.
 - __ticket_spin_unlock, which on releasing a contended lock
   (there are more cpus with tail tickets), it looks to see if the next
   cpu is blocked and wakes it if so.

When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub
functions causes all the extra code to go away.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/paravirt.h       |   30 +++-------------------
 arch/x86/include/asm/paravirt_types.h |    8 +----
 arch/x86/include/asm/spinlock.h       |   44 +++++++++++++++++++++++++++------
 arch/x86/kernel/paravirt-spinlocks.c  |   15 +---------
 arch/x86/xen/spinlock.c               |    7 ++++-
 5 files changed, 50 insertions(+), 54 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 18e3b8a..c864775 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -717,36 +717,14 @@ static inline void __set_fixmap(unsigned /* enum
fixed_addresses */ idx,
 
 #if defined(CONFIG_SMP) && defined(CONFIG_PARAVIRT_SPINLOCKS)
 
-static inline int arch_spin_is_locked(struct arch_spinlock *lock)
+static inline void __ticket_lock_spinning(struct arch_spinlock *lock, unsigned
ticket)
 {
-	return PVOP_CALL1(int, pv_lock_ops.spin_is_locked, lock);
+	PVOP_VCALL2(pv_lock_ops.lock_spinning, lock, ticket);
 }
 
-static inline int arch_spin_is_contended(struct arch_spinlock *lock)
+static inline void ____ticket_unlock_kick(struct arch_spinlock *lock, unsigned
ticket)
 {
-	return PVOP_CALL1(int, pv_lock_ops.spin_is_contended, lock);
-}
-#define arch_spin_is_contended	arch_spin_is_contended
-
-static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
-{
-	PVOP_VCALL1(pv_lock_ops.spin_lock, lock);
-}
-
-static __always_inline void arch_spin_lock_flags(struct arch_spinlock *lock,
-						  unsigned long flags)
-{
-	PVOP_VCALL2(pv_lock_ops.spin_lock_flags, lock, flags);
-}
-
-static __always_inline int arch_spin_trylock(struct arch_spinlock *lock)
-{
-	return PVOP_CALL1(int, pv_lock_ops.spin_trylock, lock);
-}
-
-static __always_inline void arch_spin_unlock(struct arch_spinlock *lock)
-{
-	PVOP_VCALL1(pv_lock_ops.spin_unlock, lock);
+	PVOP_VCALL2(pv_lock_ops.unlock_kick, lock, ticket);
 }
 
 #endif
diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
index b82bac9..1078474 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -315,12 +315,8 @@ struct pv_mmu_ops {
 
 struct arch_spinlock;
 struct pv_lock_ops {
-	int (*spin_is_locked)(struct arch_spinlock *lock);
-	int (*spin_is_contended)(struct arch_spinlock *lock);
-	void (*spin_lock)(struct arch_spinlock *lock);
-	void (*spin_lock_flags)(struct arch_spinlock *lock, unsigned long flags);
-	int (*spin_trylock)(struct arch_spinlock *lock);
-	void (*spin_unlock)(struct arch_spinlock *lock);
+	void (*lock_spinning)(struct arch_spinlock *lock, unsigned ticket);
+	void (*unlock_kick)(struct arch_spinlock *lock, unsigned ticket);
 };
 
 /* This contains all the paravirt structures: we get a convenient
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 3afb1a7..8e379d3 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -50,6 +50,21 @@ static __always_inline void __ticket_unlock_release(struct
arch_spinlock *lock)
 }
 #endif
 
+/* How long a lock should spin before we consider blocking */
+#define SPIN_THRESHOLD	(1 << 11)
+
+#ifndef CONFIG_PARAVIRT_SPINLOCKS
+
+static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
unsigned ticket)
+{
+}
+
+static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
unsigned ticket)
+{
+}
+
+#endif	/* CONFIG_PARAVIRT_SPINLOCKS */
+
 /*
  * Ticket locks are conceptually two parts, one indicating the current head of
  * the queue, and the other indicating the current tail. The lock is acquired
@@ -83,6 +98,16 @@ static __always_inline struct __raw_tickets
__ticket_spin_claim(struct arch_spin
 	return tickets;
 }
 
+/* 
+ * If a spinlock has someone waiting on it, then kick the appropriate
+ * waiting cpu.
+ */
+static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t next)
+{
+	if (unlikely(lock->tickets.tail != next))
+		____ticket_unlock_kick(lock, next);
+}
+
 static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
 {
 	register struct __raw_tickets inc;
@@ -90,10 +115,15 @@ static __always_inline void __ticket_spin_lock(struct
arch_spinlock *lock)
 	inc = __ticket_spin_claim(lock);
 
 	for (;;) {
-		if (inc.head == inc.tail)
-			goto out;
-		cpu_relax();
-		inc.head = ACCESS_ONCE(lock->tickets.head);
+		unsigned count = SPIN_THRESHOLD;
+
+		do {
+			if (inc.head == inc.tail)
+				goto out;
+			cpu_relax();
+			inc.head = ACCESS_ONCE(lock->tickets.head);
+		} while (--count);
+		__ticket_lock_spinning(lock, inc.tail);
 	}
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
@@ -114,7 +144,9 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 
 static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
 {
+	__ticket_t next = lock->tickets.head + 1;
 	__ticket_unlock_release(lock);
+	__ticket_unlock_kick(lock, next);
 	barrier();		/* prevent reordering into locked region */
 }
 
@@ -132,8 +164,6 @@ static inline int __ticket_spin_is_contended(arch_spinlock_t
*lock)
 	return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
 }
 
-#ifndef CONFIG_PARAVIRT_SPINLOCKS
-
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
 	return __ticket_spin_is_locked(lock);
@@ -166,8 +196,6 @@ static __always_inline void
arch_spin_lock_flags(arch_spinlock_t *lock,
 	arch_spin_lock(lock);
 }
 
-#endif	/* CONFIG_PARAVIRT_SPINLOCKS */
-
 static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
 {
 	while (arch_spin_is_locked(lock))
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index 676b8c7..c2e010e 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -7,21 +7,10 @@
 
 #include <asm/paravirt.h>
 
-static inline void
-default_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
-{
-	arch_spin_lock(lock);
-}
-
 struct pv_lock_ops pv_lock_ops = {
 #ifdef CONFIG_SMP
-	.spin_is_locked = __ticket_spin_is_locked,
-	.spin_is_contended = __ticket_spin_is_contended,
-
-	.spin_lock = __ticket_spin_lock,
-	.spin_lock_flags = default_spin_lock_flags,
-	.spin_trylock = __ticket_spin_trylock,
-	.spin_unlock = __ticket_spin_unlock,
+	.lock_spinning = paravirt_nop,
+	.unlock_kick = paravirt_nop,
 #endif
 };
 EXPORT_SYMBOL(pv_lock_ops);
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 23e061b..3d9da72 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -121,6 +121,9 @@ struct xen_spinlock {
 	unsigned short spinners;	/* count of waiting cpus */
 };
 
+static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
+
+#if 0
 static int xen_spin_is_locked(struct arch_spinlock *lock)
 {
 	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
@@ -148,7 +151,6 @@ static int xen_spin_trylock(struct arch_spinlock *lock)
 	return old == 0;
 }
 
-static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
 static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);
 
 /*
@@ -338,6 +340,7 @@ static void xen_spin_unlock(struct arch_spinlock *lock)
 	if (unlikely(xl->spinners))
 		xen_spin_unlock_slow(xl);
 }
+#endif
 
 static irqreturn_t dummy_handler(int irq, void *dev_id)
 {
@@ -373,12 +376,14 @@ void xen_uninit_lock_cpu(int cpu)
 
 void __init xen_init_spinlocks(void)
 {
+#if 0
 	pv_lock_ops.spin_is_locked = xen_spin_is_locked;
 	pv_lock_ops.spin_is_contended = xen_spin_is_contended;
 	pv_lock_ops.spin_lock = xen_spin_lock;
 	pv_lock_ops.spin_lock_flags = xen_spin_lock_flags;
 	pv_lock_ops.spin_trylock = xen_spin_trylock;
 	pv_lock_ops.spin_unlock = xen_spin_unlock;
+#endif
 }
 
 #ifdef CONFIG_XEN_DEBUG_FS
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 08/14] x86/ticketlock: collapse a layer of functions

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Now that the paravirtualization layer doesn't exist at the spinlock
level any more, we can collapse the __ticket_ functions into the arch_
functions.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h |   35 +++++------------------------------
 1 files changed, 5 insertions(+), 30 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 8e379d3..cfa80b5 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -108,7 +108,7 @@ static __always_inline void __ticket_unlock_kick(struct
arch_spinlock *lock, __t
 		____ticket_unlock_kick(lock, next);
 }
 
-static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
+static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
 {
 	register struct __raw_tickets inc;
 
@@ -128,7 +128,7 @@ static __always_inline void __ticket_spin_lock(struct
arch_spinlock *lock)
 out:	barrier();		/* make sure nothing creeps before the lock is taken */
 }
 
-static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
+static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
 {
 	arch_spinlock_t old, new;
 
@@ -142,7 +142,7 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
 	return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) ==
old.head_tail;
 }
 
-static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
+static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
 	__ticket_t next = lock->tickets.head + 1;
 	__ticket_unlock_release(lock);
@@ -150,46 +150,21 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
 	barrier();		/* prevent reordering into locked region */
 }
 
-static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
+static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
 	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
 	return !!(tmp.tail ^ tmp.head);
 }
 
-static inline int __ticket_spin_is_contended(arch_spinlock_t *lock)
+static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
 	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
 	return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
 }
-
-static inline int arch_spin_is_locked(arch_spinlock_t *lock)
-{
-	return __ticket_spin_is_locked(lock);
-}
-
-static inline int arch_spin_is_contended(arch_spinlock_t *lock)
-{
-	return __ticket_spin_is_contended(lock);
-}
 #define arch_spin_is_contended	arch_spin_is_contended
 
-static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
-{
-	__ticket_spin_lock(lock);
-}
-
-static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
-{
-	return __ticket_spin_trylock(lock);
-}
-
-static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
-{
-	__ticket_spin_unlock(lock);
-}
-
 static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
 						  unsigned long flags)
 {
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 09/14] xen/pvticketlock: Xen implementation for PV ticket locks

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Replace the old Xen implementation of PV spinlocks with and implementation
of xen_lock_spinning and xen_unlock_kick.

xen_lock_spinning simply registers the cpu in its entry in lock_waiting,
adds itself to the waiting_cpus set, and blocks on an event channel
until the channel becomes pending.

xen_unlock_kick searches the cpus in waiting_cpus looking for the one
which next wants this lock with the next ticket, if any.  If found,
it kicks it by making its event channel pending, which wakes it up.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/xen/spinlock.c |  281 ++++++-----------------------------------------
 1 files changed, 36 insertions(+), 245 deletions(-)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 3d9da72..5feb897 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -19,32 +19,21 @@
 #ifdef CONFIG_XEN_DEBUG_FS
 static struct xen_spinlock_stats
 {
-	u64 taken;
 	u32 taken_slow;
-	u32 taken_slow_nested;
 	u32 taken_slow_pickup;
 	u32 taken_slow_spurious;
-	u32 taken_slow_irqenable;
 
-	u64 released;
 	u32 released_slow;
 	u32 released_slow_kicked;
 
 #define HISTO_BUCKETS	30
-	u32 histo_spin_total[HISTO_BUCKETS+1];
-	u32 histo_spin_spinning[HISTO_BUCKETS+1];
 	u32 histo_spin_blocked[HISTO_BUCKETS+1];
 
-	u64 time_total;
-	u64 time_spinning;
 	u64 time_blocked;
 } spinlock_stats;
 
 static u8 zero_stats;
 
-static unsigned lock_timeout = 1 << 10;
-#define TIMEOUT lock_timeout
-
 static inline void check_zero(void)
 {
 	if (unlikely(zero_stats)) {
@@ -73,22 +62,6 @@ static void __spin_time_accum(u64 delta, u32 *array)
 		array[HISTO_BUCKETS]++;
 }
 
-static inline void spin_time_accum_spinning(u64 start)
-{
-	u32 delta = xen_clocksource_read() - start;
-
-	__spin_time_accum(delta, spinlock_stats.histo_spin_spinning);
-	spinlock_stats.time_spinning += delta;
-}
-
-static inline void spin_time_accum_total(u64 start)
-{
-	u32 delta = xen_clocksource_read() - start;
-
-	__spin_time_accum(delta, spinlock_stats.histo_spin_total);
-	spinlock_stats.time_total += delta;
-}
-
 static inline void spin_time_accum_blocked(u64 start)
 {
 	u32 delta = xen_clocksource_read() - start;
@@ -105,214 +78,76 @@ static inline u64 spin_time_start(void)
 	return 0;
 }
 
-static inline void spin_time_accum_total(u64 start)
-{
-}
-static inline void spin_time_accum_spinning(u64 start)
-{
-}
 static inline void spin_time_accum_blocked(u64 start)
 {
 }
 #endif  /* CONFIG_XEN_DEBUG_FS */
 
-struct xen_spinlock {
-	unsigned char lock;		/* 0 -> free; 1 -> locked */
-	unsigned short spinners;	/* count of waiting cpus */
+struct xen_lock_waiting {
+	struct arch_spinlock *lock;
+	__ticket_t want;
 };
 
 static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
+static DEFINE_PER_CPU(struct xen_lock_waiting, lock_waiting);
+static cpumask_t waiting_cpus;
 
-#if 0
-static int xen_spin_is_locked(struct arch_spinlock *lock)
-{
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-
-	return xl->lock != 0;
-}
-
-static int xen_spin_is_contended(struct arch_spinlock *lock)
-{
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-
-	/* Not strictly true; this is only the count of contended
-	   lock-takers entering the slow path. */
-	return xl->spinners != 0;
-}
-
-static int xen_spin_trylock(struct arch_spinlock *lock)
-{
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-	u8 old = 1;
-
-	asm("xchgb %b0,%1"
-	    : "+q" (old), "+m" (xl->lock) : :
"memory");
-
-	return old == 0;
-}
-
-static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);
-
-/*
- * Mark a cpu as interested in a lock.  Returns the CPU's previous
- * lock of interest, in case we got preempted by an interrupt.
- */
-static inline struct xen_spinlock *spinning_lock(struct xen_spinlock *xl)
-{
-	struct xen_spinlock *prev;
-
-	prev = __get_cpu_var(lock_spinners);
-	__get_cpu_var(lock_spinners) = xl;
-
-	wmb();			/* set lock of interest before count */
-
-	asm(LOCK_PREFIX " incw %0"
-	    : "+m" (xl->spinners) : : "memory");
-
-	return prev;
-}
-
-/*
- * Mark a cpu as no longer interested in a lock.  Restores previous
- * lock of interest (NULL for none).
- */
-static inline void unspinning_lock(struct xen_spinlock *xl, struct xen_spinlock
*prev)
-{
-	asm(LOCK_PREFIX " decw %0"
-	    : "+m" (xl->spinners) : : "memory");
-	wmb();			/* decrement count before restoring lock */
-	__get_cpu_var(lock_spinners) = prev;
-}
-
-static noinline int xen_spin_lock_slow(struct arch_spinlock *lock, bool
irq_enable)
+static void xen_lock_spinning(struct arch_spinlock *lock, unsigned want)
 {
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-	struct xen_spinlock *prev;
 	int irq = __get_cpu_var(lock_kicker_irq);
-	int ret;
+	struct xen_lock_waiting *w = &__get_cpu_var(lock_waiting);
+	int cpu = smp_processor_id();
 	u64 start;
 
 	/* If kicker interrupts not initialized yet, just spin */
 	if (irq == -1)
-		return 0;
+		return;
 
 	start = spin_time_start();
 
-	/* announce we're spinning */
-	prev = spinning_lock(xl);
+	w->want = want;
+	w->lock = lock;
+
+	/* This uses set_bit, which atomic and therefore a barrier */
+	cpumask_set_cpu(cpu, &waiting_cpus);
 
 	ADD_STATS(taken_slow, 1);
-	ADD_STATS(taken_slow_nested, prev != NULL);
-
-	do {
-		unsigned long flags;
-
-		/* clear pending */
-		xen_clear_irq_pending(irq);
-
-		/* check again make sure it didn't become free while
-		   we weren't looking  */
-		ret = xen_spin_trylock(lock);
-		if (ret) {
-			ADD_STATS(taken_slow_pickup, 1);
-
-			/*
-			 * If we interrupted another spinlock while it
-			 * was blocking, make sure it doesn't block
-			 * without rechecking the lock.
-			 */
-			if (prev != NULL)
-				xen_set_irq_pending(irq);
-			goto out;
-		}
 
-		flags = arch_local_save_flags();
-		if (irq_enable) {
-			ADD_STATS(taken_slow_irqenable, 1);
-			raw_local_irq_enable();
-		}
+	/* clear pending */
+	xen_clear_irq_pending(irq);
 
-		/*
-		 * Block until irq becomes pending.  If we're
-		 * interrupted at this point (after the trylock but
-		 * before entering the block), then the nested lock
-		 * handler guarantees that the irq will be left
-		 * pending if there's any chance the lock became free;
-		 * xen_poll_irq() returns immediately if the irq is
-		 * pending.
-		 */
-		xen_poll_irq(irq);
+	/* Only check lock once pending cleared */
+	barrier();
 
-		raw_local_irq_restore(flags);
+	/* check again make sure it didn't become free while
+	   we weren't looking  */
+	if (ACCESS_ONCE(lock->tickets.head) == want) {
+		ADD_STATS(taken_slow_pickup, 1);
+		goto out;
+	}
 
-		ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
-	} while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */
+	/* Block until irq becomes pending (or perhaps a spurious wakeup) */
+	xen_poll_irq(irq);
+	ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
 
 	kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));
 
 out:
-	unspinning_lock(xl, prev);
+	cpumask_clear_cpu(cpu, &waiting_cpus);
+	w->lock = NULL;
 	spin_time_accum_blocked(start);
-
-	return ret;
 }
 
-static inline void __xen_spin_lock(struct arch_spinlock *lock, bool irq_enable)
-{
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-	unsigned timeout;
-	u8 oldval;
-	u64 start_spin;
-
-	ADD_STATS(taken, 1);
-
-	start_spin = spin_time_start();
-
-	do {
-		u64 start_spin_fast = spin_time_start();
-
-		timeout = TIMEOUT;
-
-		asm("1: xchgb %1,%0\n"
-		    "   testb %1,%1\n"
-		    "   jz 3f\n"
-		    "2: rep;nop\n"
-		    "   cmpb $0,%0\n"
-		    "   je 1b\n"
-		    "   dec %2\n"
-		    "   jnz 2b\n"
-		    "3:\n"
-		    : "+m" (xl->lock), "=q" (oldval), "+r"
(timeout)
-		    : "1" (1)
-		    : "memory");
-
-		spin_time_accum_spinning(start_spin_fast);
-
-	} while (unlikely(oldval != 0 &&
-			  (TIMEOUT == ~0 || !xen_spin_lock_slow(lock, irq_enable))));
-
-	spin_time_accum_total(start_spin);
-}
-
-static void xen_spin_lock(struct arch_spinlock *lock)
-{
-	__xen_spin_lock(lock, false);
-}
-
-static void xen_spin_lock_flags(struct arch_spinlock *lock, unsigned long
flags)
-{
-	__xen_spin_lock(lock, !raw_irqs_disabled_flags(flags));
-}
-
-static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
+static void xen_unlock_kick(struct arch_spinlock *lock, unsigned next)
 {
 	int cpu;
 
 	ADD_STATS(released_slow, 1);
 
-	for_each_online_cpu(cpu) {
-		/* XXX should mix up next cpu selection */
-		if (per_cpu(lock_spinners, cpu) == xl) {
+	for_each_cpu(cpu, &waiting_cpus) {
+		const struct xen_lock_waiting *w = &per_cpu(lock_waiting, cpu);
+
+		if (w->lock == lock && w->want == next) {
 			ADD_STATS(released_slow_kicked, 1);
 			xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
 			break;
@@ -320,28 +155,6 @@ static noinline void xen_spin_unlock_slow(struct
xen_spinlock *xl)
 	}
 }
 
-static void xen_spin_unlock(struct arch_spinlock *lock)
-{
-	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
-
-	ADD_STATS(released, 1);
-
-	smp_wmb();		/* make sure no writes get moved after unlock */
-	xl->lock = 0;		/* release lock */
-
-	/*
-	 * Make sure unlock happens before checking for waiting
-	 * spinners.  We need a strong barrier to enforce the
-	 * write-read ordering to different memory locations, as the
-	 * CPU makes no implied guarantees about their ordering.
-	 */
-	mb();
-
-	if (unlikely(xl->spinners))
-		xen_spin_unlock_slow(xl);
-}
-#endif
-
 static irqreturn_t dummy_handler(int irq, void *dev_id)
 {
 	BUG();
@@ -376,14 +189,8 @@ void xen_uninit_lock_cpu(int cpu)
 
 void __init xen_init_spinlocks(void)
 {
-#if 0
-	pv_lock_ops.spin_is_locked = xen_spin_is_locked;
-	pv_lock_ops.spin_is_contended = xen_spin_is_contended;
-	pv_lock_ops.spin_lock = xen_spin_lock;
-	pv_lock_ops.spin_lock_flags = xen_spin_lock_flags;
-	pv_lock_ops.spin_trylock = xen_spin_trylock;
-	pv_lock_ops.spin_unlock = xen_spin_unlock;
-#endif
+	pv_lock_ops.lock_spinning = xen_lock_spinning;
+	pv_lock_ops.unlock_kick = xen_unlock_kick;
 }
 
 #ifdef CONFIG_XEN_DEBUG_FS
@@ -401,37 +208,21 @@ static int __init xen_spinlock_debugfs(void)
 
 	debugfs_create_u8("zero_stats", 0644, d_spin_debug,
&zero_stats);
 
-	debugfs_create_u32("timeout", 0644, d_spin_debug,
&lock_timeout);
-
-	debugfs_create_u64("taken", 0444, d_spin_debug,
&spinlock_stats.taken);
 	debugfs_create_u32("taken_slow", 0444, d_spin_debug,
 			   &spinlock_stats.taken_slow);
-	debugfs_create_u32("taken_slow_nested", 0444, d_spin_debug,
-			   &spinlock_stats.taken_slow_nested);
 	debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
 			   &spinlock_stats.taken_slow_pickup);
 	debugfs_create_u32("taken_slow_spurious", 0444, d_spin_debug,
 			   &spinlock_stats.taken_slow_spurious);
-	debugfs_create_u32("taken_slow_irqenable", 0444, d_spin_debug,
-			   &spinlock_stats.taken_slow_irqenable);
 
-	debugfs_create_u64("released", 0444, d_spin_debug,
&spinlock_stats.released);
 	debugfs_create_u32("released_slow", 0444, d_spin_debug,
 			   &spinlock_stats.released_slow);
 	debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
 			   &spinlock_stats.released_slow_kicked);
 
-	debugfs_create_u64("time_spinning", 0444, d_spin_debug,
-			   &spinlock_stats.time_spinning);
 	debugfs_create_u64("time_blocked", 0444, d_spin_debug,
 			   &spinlock_stats.time_blocked);
-	debugfs_create_u64("time_total", 0444, d_spin_debug,
-			   &spinlock_stats.time_total);
 
-	xen_debugfs_create_u32_array("histo_total", 0444, d_spin_debug,
-				     spinlock_stats.histo_spin_total, HISTO_BUCKETS + 1);
-	xen_debugfs_create_u32_array("histo_spinning", 0444, d_spin_debug,
-				     spinlock_stats.histo_spin_spinning, HISTO_BUCKETS + 1);
 	xen_debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
 				     spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);
 
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 10/14] x86/pvticketlock: use callee-save for lock_spinning

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Although the lock_spinning calls in the spinlock code are on the
uncommon path, their presence can cause the compiler to generate many
more register save/restores in the function pre/postamble, which is in
the fast path.  To avoid this, convert it to using the pvops callee-save
calling convention, which defers all the save/restores until the actual
function is called, keeping the fastpath clean.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/paravirt.h       |    2 +-
 arch/x86/include/asm/paravirt_types.h |    2 +-
 arch/x86/kernel/paravirt-spinlocks.c  |    2 +-
 arch/x86/xen/spinlock.c               |    3 ++-
 4 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index c864775..6f275ca 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -719,7 +719,7 @@ static inline void __set_fixmap(unsigned /* enum
fixed_addresses */ idx,
 
 static inline void __ticket_lock_spinning(struct arch_spinlock *lock, unsigned
ticket)
 {
-	PVOP_VCALL2(pv_lock_ops.lock_spinning, lock, ticket);
+	PVOP_VCALLEE2(pv_lock_ops.lock_spinning, lock, ticket);
 }
 
 static inline void ____ticket_unlock_kick(struct arch_spinlock *lock, unsigned
ticket)
diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
index 1078474..53f249a 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -315,7 +315,7 @@ struct pv_mmu_ops {
 
 struct arch_spinlock;
 struct pv_lock_ops {
-	void (*lock_spinning)(struct arch_spinlock *lock, unsigned ticket);
+	struct paravirt_callee_save lock_spinning;
 	void (*unlock_kick)(struct arch_spinlock *lock, unsigned ticket);
 };
 
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index c2e010e..4251c1d 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -9,7 +9,7 @@
 
 struct pv_lock_ops pv_lock_ops = {
 #ifdef CONFIG_SMP
-	.lock_spinning = paravirt_nop,
+	.lock_spinning = __PV_IS_CALLEE_SAVE(paravirt_nop),
 	.unlock_kick = paravirt_nop,
 #endif
 };
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 5feb897..c31c5a3 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -137,6 +137,7 @@ out:
 	w->lock = NULL;
 	spin_time_accum_blocked(start);
 }
+PV_CALLEE_SAVE_REGS_THUNK(xen_lock_spinning);
 
 static void xen_unlock_kick(struct arch_spinlock *lock, unsigned next)
 {
@@ -189,7 +190,7 @@ void xen_uninit_lock_cpu(int cpu)
 
 void __init xen_init_spinlocks(void)
 {
-	pv_lock_ops.lock_spinning = xen_lock_spinning;
+	pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(xen_lock_spinning);
 	pv_lock_ops.unlock_kick = xen_unlock_kick;
 }
 
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 11/14] x86/ticketlock: don't inline _spin_unlock when using paravirt spinlocks

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

The code size expands somewhat, and its probably better to just call
a function rather than inline it.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/Kconfig     |    3 +++
 kernel/Kconfig.locks |    2 +-
 2 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e832768..a615c9c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -531,6 +531,9 @@ config PARAVIRT_SPINLOCKS
 
 	  If you are unsure how to answer this question, answer N.
 
+config ARCH_NOINLINE_SPIN_UNLOCK
+       def_bool PARAVIRT_SPINLOCKS
+
 config PARAVIRT_CLOCK
 	bool
 
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index 88c92fb..3216c22 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -125,7 +125,7 @@ config INLINE_SPIN_LOCK_IRQSAVE
 		 ARCH_INLINE_SPIN_LOCK_IRQSAVE
 
 config INLINE_SPIN_UNLOCK
-	def_bool !DEBUG_SPINLOCK && (!PREEMPT || ARCH_INLINE_SPIN_UNLOCK)
+	def_bool !DEBUG_SPINLOCK && (!PREEMPT || ARCH_INLINE_SPIN_UNLOCK)
&& !ARCH_NOINLINE_SPIN_UNLOCK
 
 config INLINE_SPIN_UNLOCK_BH
 	def_bool !DEBUG_SPINLOCK && ARCH_INLINE_SPIN_UNLOCK_BH
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 12/14] x86/ticketlocks: when paravirtualizing ticket locks, increment by 2

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Increment ticket head/tails by 2 rather than 1 to leave the LSB free
to store a "is in slowpath state" bit.  This halves the number
of possible CPUs for a given ticket size, but this shouldn't matter
in practice - kernels built for 32k+ CPU systems are probably
specially built for the hardware rather than a generic distro
kernel.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h       |   18 +++++++++---------
 arch/x86/include/asm/spinlock_types.h |   10 +++++++++-
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index cfa80b5..9e1c7ce 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -36,17 +36,17 @@
 static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
 {
 	if (sizeof(lock->tickets.head) == sizeof(u8))
-		asm (LOCK_PREFIX "incb %0"
-		     : "+m" (lock->tickets.head) : : "memory");
+		asm (LOCK_PREFIX "addb %1, %0"
+		     : "+m" (lock->tickets.head) : "i"
(TICKET_LOCK_INC) : "memory");
 	else
-		asm (LOCK_PREFIX "incw %0"
-		     : "+m" (lock->tickets.head) : : "memory");
+		asm (LOCK_PREFIX "addw %1, %0"
+		     : "+m" (lock->tickets.head) : "i"
(TICKET_LOCK_INC) : "memory");
 
 }
 #else
 static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
 {
-	lock->tickets.head++;
+	lock->tickets.head += TICKET_LOCK_INC;
 }
 #endif
 
@@ -84,7 +84,7 @@ static __always_inline void ____ticket_unlock_kick(struct
arch_spinlock *lock, u
  */
 static __always_inline struct __raw_tickets __ticket_spin_claim(struct
arch_spinlock *lock)
 {
-	register struct __raw_tickets tickets = { .tail = 1 };
+	register struct __raw_tickets tickets = { .tail = TICKET_LOCK_INC };
 
 	if (sizeof(lock->tickets.head) == sizeof(u8))
 		asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
@@ -136,7 +136,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t
*lock)
 	if (old.tickets.head != old.tickets.tail)
 		return 0;
 
-	new.head_tail = old.head_tail + (1 << TICKET_SHIFT);
+	new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
 
 	/* cmpxchg is a full barrier, so nothing can move before it */
 	return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) ==
old.head_tail;
@@ -144,7 +144,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t
*lock)
 
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
-	__ticket_t next = lock->tickets.head + 1;
+	__ticket_t next = lock->tickets.head + TICKET_LOCK_INC;
 	__ticket_unlock_release(lock);
 	__ticket_unlock_kick(lock, next);
 	barrier();		/* prevent reordering into locked region */
@@ -161,7 +161,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t
*lock)
 {
 	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
 
-	return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
+	return ((tmp.tail - tmp.head) & TICKET_MASK) > TICKET_LOCK_INC;
 }
 #define arch_spin_is_contended	arch_spin_is_contended
 
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index 72e154e..0553c0b 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -7,7 +7,13 @@
 
 #include <linux/types.h>
 
-#if (CONFIG_NR_CPUS < 256)
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define __TICKET_LOCK_INC	2
+#else
+#define __TICKET_LOCK_INC	1
+#endif
+
+#if (CONFIG_NR_CPUS < (256 / __TICKET_LOCK_INC))
 typedef u8  __ticket_t;
 typedef u16 __ticketpair_t;
 #else
@@ -15,6 +21,8 @@ typedef u16 __ticket_t;
 typedef u32 __ticketpair_t;
 #endif
 
+#define TICKET_LOCK_INC	((__ticket_t)__TICKET_LOCK_INC)
+
 #define TICKET_SHIFT	(sizeof(__ticket_t) * 8)
 #define TICKET_MASK	((__ticket_t)((1 << TICKET_SHIFT) - 1))
 
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 13/14] x86/ticketlock: add slowpath logic

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

Maintain a flag in both LSBs of the ticket lock which indicates whether
anyone is in the lock slowpath and may need kicking when the current
holder unlocks.  The flags are set when the first locker enters
the slowpath, and cleared when unlocking to an empty queue.

In the specific implementation of lock_spinning(), make sure to set
the slowpath flags on the lock just before blocking.  We must do
this before the last-chance pickup test to prevent a deadlock
with the unlocker:

Unlocker			Locker
				test for lock pickup
					-> fail
test slowpath + unlock
	-> false
				set slowpath flags
				block

Whereas this works in any ordering:

Unlocker			Locker
				set slowpath flags
				test for lock pickup
					-> fail
				block
test slowpath + unlock
	-> true, kick

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/spinlock.h       |   53 ++++++++++++++++++++++++++++-----
 arch/x86/include/asm/spinlock_types.h |    2 +
 arch/x86/kernel/paravirt-spinlocks.c  |   37 +++++++++++++++++++++++
 arch/x86/xen/spinlock.c               |    4 ++
 4 files changed, 88 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 9e1c7ce..8d1cb42 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -53,7 +53,38 @@ static __always_inline void __ticket_unlock_release(struct
arch_spinlock *lock)
 /* How long a lock should spin before we consider blocking */
 #define SPIN_THRESHOLD	(1 << 11)
 
-#ifndef CONFIG_PARAVIRT_SPINLOCKS
+/* Only defined when CONFIG_PARAVIRT_SPINLOCKS defined, but may as
+ * well leave the prototype always visible.  */
+extern void __ticket_unlock_release_slowpath(struct arch_spinlock *lock);
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+
+/*
+ * Return true if someone is in the slowpath on this lock.  This
+ * should only be used by the current lock-holder.
+ */
+static inline bool __ticket_in_slowpath(struct arch_spinlock *lock)
+{
+	return !!(lock->tickets.tail & TICKET_SLOWPATH_FLAG);
+}
+
+static inline void __ticket_enter_slowpath(struct arch_spinlock *lock)
+{
+	if (sizeof(lock->tickets.tail) == sizeof(u8))
+		asm (LOCK_PREFIX "orb %1, %0"
+		     : "+m" (lock->tickets.tail)
+		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
+	else
+		asm (LOCK_PREFIX "orw %1, %0"
+		     : "+m" (lock->tickets.tail)
+		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
+}
+
+#else  /* !CONFIG_PARAVIRT_SPINLOCKS */
+static inline bool __ticket_in_slowpath(struct arch_spinlock *lock)
+{
+	return false;
+}
 
 static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
unsigned ticket)
 {
@@ -84,18 +115,22 @@ static __always_inline void ____ticket_unlock_kick(struct
arch_spinlock *lock, u
  */
 static __always_inline struct __raw_tickets __ticket_spin_claim(struct
arch_spinlock *lock)
 {
-	register struct __raw_tickets tickets = { .tail = TICKET_LOCK_INC };
+	register struct arch_spinlock tickets = {
+		{ .tickets.tail = TICKET_LOCK_INC }
+	};
 
 	if (sizeof(lock->tickets.head) == sizeof(u8))
 		asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
-			      : "+r" (tickets), "+m" (lock->tickets)
+			      : "+r" (tickets.head_tail), "+m"
(lock->tickets)
 			      : : "memory", "cc");
 	else
 		asm volatile (LOCK_PREFIX "xaddl %0, %1\n"
-			     : "+r" (tickets), "+m" (lock->tickets)
+			     : "+r" (tickets.head_tail), "+m" (lock->tickets)
 			     : : "memory", "cc");
 
-	return tickets;
+	tickets.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+
+	return tickets.tickets;
 }
 
 /* 
@@ -144,9 +179,11 @@ static __always_inline int
arch_spin_trylock(arch_spinlock_t *lock)
 
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
-	__ticket_t next = lock->tickets.head + TICKET_LOCK_INC;
-	__ticket_unlock_release(lock);
-	__ticket_unlock_kick(lock, next);
+	barrier();		/* prevent reordering out of locked region */
+	if (unlikely(__ticket_in_slowpath(lock)))
+		__ticket_unlock_release_slowpath(lock);
+	else
+		__ticket_unlock_release(lock);
 	barrier();		/* prevent reordering into locked region */
 }
 
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index 0553c0b..7b383e2 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -9,8 +9,10 @@
 
 #ifdef CONFIG_PARAVIRT_SPINLOCKS
 #define __TICKET_LOCK_INC	2
+#define TICKET_SLOWPATH_FLAG	((__ticket_t)1)
 #else
 #define __TICKET_LOCK_INC	1
+#define TICKET_SLOWPATH_FLAG	((__ticket_t)0)
 #endif
 
 #if (CONFIG_NR_CPUS < (256 / __TICKET_LOCK_INC))
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index 4251c1d..21b6986 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -15,3 +15,40 @@ struct pv_lock_ops pv_lock_ops = {
 };
 EXPORT_SYMBOL(pv_lock_ops);
 
+
+/*
+ * If we're unlocking and we're leaving the lock uncontended
(there's
+ * nobody else waiting for the lock), then we can clear the slowpath
+ * bits.  However, we need to be careful about this because someone
+ * may just be entering as we leave, and enter the slowpath.
+ */
+void __ticket_unlock_release_slowpath(struct arch_spinlock *lock)
+{
+	struct arch_spinlock old, new;
+
+	BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
+
+	old = ACCESS_ONCE(*lock);
+
+	new = old;
+	new.tickets.head += TICKET_LOCK_INC;
+
+	/* Clear the slowpath flag */
+	new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+
+	/*
+	 * If there's currently people waiting or someone snuck in
+	 * since we read the lock above, then do a normal unlock and
+	 * kick.  If we managed to unlock with no queued waiters, then
+	 * we can clear the slowpath flag.
+	 */
+	if (new.tickets.head != new.tickets.tail ||
+	    cmpxchg(&lock->head_tail,
+		    old.head_tail, new.head_tail) != old.head_tail) {
+		/* still people waiting */
+		__ticket_unlock_release(lock);
+	}
+
+	__ticket_unlock_kick(lock, new.tickets.head);
+}
+EXPORT_SYMBOL(__ticket_unlock_release_slowpath);
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index c31c5a3..91f2fd2 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -119,6 +119,10 @@ static void xen_lock_spinning(struct arch_spinlock *lock,
unsigned want)
 	/* Only check lock once pending cleared */
 	barrier();
 
+	/* Mark entry to slowpath before doing the pickup test to make
+	   sure we don't deadlock with an unlocker. */
+	__ticket_enter_slowpath(lock);
+
 	/* check again make sure it didn't become free while
 	   we weren't looking  */
 	if (ACCESS_ONCE(lock->tickets.head) == want) {
-- 
1.7.2.3

Jeremy Fitzhardinge

2010-Nov-16 21:08 UTC

head link

[PATCH 14/14] x86/ticketlocks: tidy up __ticket_unlock_kick()

From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>

__ticket_unlock_kick() is now only called from known slowpaths, so there's
no need for it to do any checking of its own.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
---
 arch/x86/include/asm/paravirt.h |    2 +-
 arch/x86/include/asm/spinlock.h |   14 --------------
 2 files changed, 1 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 6f275ca..7755b16 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -722,7 +722,7 @@ static inline void __ticket_lock_spinning(struct
arch_spinlock *lock, unsigned t
 	PVOP_VCALLEE2(pv_lock_ops.lock_spinning, lock, ticket);
 }
 
-static inline void ____ticket_unlock_kick(struct arch_spinlock *lock, unsigned
ticket)
+static inline void __ticket_unlock_kick(struct arch_spinlock *lock, unsigned
ticket)
 {
 	PVOP_VCALL2(pv_lock_ops.unlock_kick, lock, ticket);
 }
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 8d1cb42..70675bc 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -90,10 +90,6 @@ static __always_inline void __ticket_lock_spinning(struct
arch_spinlock *lock, u
 {
 }
 
-static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
unsigned ticket)
-{
-}
-
 #endif	/* CONFIG_PARAVIRT_SPINLOCKS */
 
 /*
@@ -133,16 +129,6 @@ static __always_inline struct __raw_tickets
__ticket_spin_claim(struct arch_spin
 	return tickets.tickets;
 }
 
-/* 
- * If a spinlock has someone waiting on it, then kick the appropriate
- * waiting cpu.
- */
-static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t next)
-{
-	if (unlikely(lock->tickets.tail != next))
-		____ticket_unlock_kick(lock, next);
-}
-
 static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
 {
 	register struct __raw_tickets inc;
-- 
1.7.2.3

Jan Beulich

2010-Nov-17 08:31 UTC

head link

[PATCH 13/14] x86/ticketlock: add slowpath logic

>>> On 16.11.10 at 22:08, Jeremy Fitzhardinge <jeremy at
goop.org> wrote:
> +static inline void __ticket_enter_slowpath(struct arch_spinlock *lock)
> +{
> +	if (sizeof(lock->tickets.tail) == sizeof(u8))
> +		asm (LOCK_PREFIX "orb %1, %0"
> +		     : "+m" (lock->tickets.tail)
> +		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
> +	else
> +		asm (LOCK_PREFIX "orw %1, %0"
> +		     : "+m" (lock->tickets.tail)
> +		     : "i" (TICKET_SLOWPATH_FLAG) : "memory");
> +}
Came only now to mind: Here and elsewhere, did you try using
%z0 to have gcc produce the opcode suffix character, rather
than having these somewhat ugly if()-s?

Jan

Avi Kivity

2010-Nov-17 08:56 UTC

head link

[PATCH 00/14] PV ticket locks without expanding spinlock

On 11/16/2010 11:08 PM, Jeremy Fitzhardinge wrote:> From: Jeremy Fitzhardinge<jeremy.fitzhardinge at citrix.com>
>
> Hi all,
>
> This is a revised version of the pvticket lock series.
>
> The early part of the series is mostly unchanged: it converts the bulk
> of the ticket lock code into C and makes the "small" and
"large"
> ticket code common.  The only changes are the incorporation of various
> review comments.
>
> The latter part of the series converts from pv spinlocks to pv ticket
> locks (ie, using the ticket lock fastpath as-is, but adding pv ops for
> the ticketlock slowpaths).
>
> The significant difference here is that rather than adding a new
> ticket_t-sized element to arch_spinlock_t - effectively doubling the
> size - I steal the LSB of the tickets themselves to store a bit.  This
> allows the structure to remain the same size, but at the cost of
> halving the max number of CPUs (127 for a 8-bit ticket, and a hard max
> of 32767 overall).
>
> The extra bit (well, two, but one is unused) in indicates whether the
> lock has gone into "slowpath state", which means one of its
lockers
> has entered its slowpath and has blocked in the hypervisor.  This
> means the current lock-holder needs to make sure it gets kicked out of
> the hypervisor on unlock.
>
> The spinlock remains in slowpath state until the last unlock happens
> (ie there are no more queued lockers).
>
> This code survives for a while with moderate testing, (make -j 100 on
> 8 VCPUs on a 4 PCPU system), but locks up after about 20 iterations,
> so there's still some race/deadlock in there (probably something
> misordered), but I think the basic approach is sound.
This is going to be very useful for kvm; I'd like to see the fixed 
version gets merged.

-- 
error compiling committee.c: too many arguments to function

Peter Zijlstra

2010-Nov-17 12:21 UTC

head link

[PATCH 13/14] x86/ticketlock: add slowpath logic

On Tue, 2010-11-16 at 13:08 -0800, Jeremy Fitzhardinge
wrote:> Maintain a flag in both LSBs of the ticket lock which indicates whether
> anyone is in the lock slowpath and may need kicking when the current
> holder unlocks.  The flags are set when the first locker enters
> the slowpath, and cleared when unlocking to an empty queue.
So here you say you set both LSBs in order to keep head == tail working,
but the code seems to suggest you only use the tail LSB.

I think I see why using only one LSB is sufficient, but some consistency
would be nice :-)

Konrad Rzeszutek Wilk

2011-Jan-11 17:21 UTC

head link

[Xen-devel] [PATCH 01/14] x86/ticketlock: clean up types and accessors

>  static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
>  {
> -	int tmp = ACCESS_ONCE(lock->slock);
> +	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
>  
> -	return !!(((tmp >> TICKET_SHIFT) ^ tmp) & ((1 <<
TICKET_SHIFT) - 1));
> +	return !!(tmp.tail ^ tmp.head);
Does it make sense to mask it here it here with TICKET_MASK as it was done
before?

Srivatsa Vaddagiri

2011-Jan-17 15:22 UTC

head link

[PATCH 13/14] x86/ticketlock: add slowpath logic

On Tue, Nov 16, 2010 at 01:08:44PM -0800, Jeremy Fitzhardinge
wrote:> From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
> 
> Maintain a flag in both LSBs of the ticket lock which indicates whether
> anyone is in the lock slowpath and may need kicking when the current
> holder unlocks.  The flags are set when the first locker enters
> the slowpath, and cleared when unlocking to an empty queue.
> 
> In the specific implementation of lock_spinning(), make sure to set
> the slowpath flags on the lock just before blocking.  We must do
> this before the last-chance pickup test to prevent a deadlock
> with the unlocker:
> 
> Unlocker			Locker
> 				test for lock pickup
> 					-> fail
> test slowpath + unlock
> 	-> false
> 				set slowpath flags
> 				block
> 
> Whereas this works in any ordering:
> 
> Unlocker			Locker
> 				set slowpath flags
> 				test for lock pickup
> 					-> fail
> 				block
> test slowpath + unlock
> 	-> true, kick
I think this is still racy ..

Unlocker				Locker

				
test slowpath
	-> false
		
				set slowpath flag
				test for lock pickup
					-> fail
				block


unlock

unlock needs to happen first before testing slowpath? I have made that change
for my KVM guest and it seems to be working well with that change .. Will
cleanup and post my patches shortly

- vatsa

Srivatsa Vaddagiri

2011-Jan-19 16:44 UTC

head link

[PATCH 00/14] PV ticket locks without expanding spinlock

On Tue, Nov 16, 2010 at 01:08:31PM -0800, Jeremy Fitzhardinge
wrote:> From: Jeremy Fitzhardinge <jeremy.fitzhardinge at citrix.com>
> 
> Hi all,
> 
> This is a revised version of the pvticket lock series.
The 3-patch series to follow this email extends KVM-hypervisor and Linux guest 
running on KVM-hypervisor to support pv-ticket spinlocks.

Two hypercalls are being introduced in KVM hypervisor, one that allows a
vcpu (spinning on a lock) to block and another that allows a vcpu to kick
another out of blocking state.

Patches are against 2.6.37 mainline kernel. I also don't yet have numbers 
at this time to show benefit of pv-ticketlocks - I would think the benefit
should be similar across hypervisors (Xen and KVM in this case).

- vatsa

Peter Zijlstra

2011-Jan-19 17:50 UTC

head link

[PATCH 2/3] kvm hypervisor : Add hypercalls to support pv-ticketlock

On Wed, 2011-01-19 at 22:53 +0530, Srivatsa Vaddagiri
wrote:> On Wed, Jan 19, 2011 at 10:42:39PM +0530, Srivatsa Vaddagiri wrote:
> > Add two hypercalls to KVM hypervisor to support pv-ticketlocks.
> > 
> > KVM_HC_WAIT_FOR_KICK blocks the calling vcpu until another vcpu kicks
it or it
> > is woken up because of an event like interrupt.
> 
> One possibility is to extend this hypercall to do a directed yield as well,
> which needs some more thought. Another issue that needs to be resolved with
> pv-ticketlocks is the impact on intra-VM fairness. A guest experiencing
heavy
> contention can keep yielding cpu, allowing other VMs to get more time than
they
> deserve.
No, yield sucks, if you know the owner you can do actual PI.

Jan Beulich

2011-Jan-20 09:52 UTC

head link

[PATCH 13/14] x86/ticketlock: add slowpath logic

>>> On 19.01.11 at 19:55, Jeremy Fitzhardinge <jeremy at
goop.org> wrote:
> On 01/19/2011 10:39 AM, Srivatsa Vaddagiri wrote:
>> I have tested quite extensively with booting a 16-vcpu guest (on a
16-pcpu
> host)
>> and running kernel compine (with 32-threads). Without this patch, I had
>> difficulty booting/shutting-down successfully (it would hang mid-way).
> 
> Sounds good.  But I like to test with "make -j 100-200" to really
give
> things a workout.
Hmm, in my experience, heavily over-committing CPUs (e.g. a
domain with double or more the vCPU-s that the system has
pCPU-s, or the pCPU-s the domain is allowed to run on) is a
much better test for eventual problems in the spin lock code
paths.

Jan

Jeremy Fitzhardinge

2011-Jan-20 17:49 UTC

head link

[PATCH 2/3] kvm hypervisor : Add hypercalls to support pv-ticketlock

On 01/20/2011 03:42 AM, Srivatsa Vaddagiri wrote:> On Wed, Jan 19, 2011 at 10:53:52AM -0800, Jeremy Fitzhardinge wrote:
>>> The reason for wanting this should be clear I guess, it allows PI.
>> Well, if we can expand the spinlock to include an owner, then all this
>> becomes moot...
> How so? Having an owner will not eliminate the need for pv-ticketlocks
> afaict. We still need a mechanism to reduce latency in scheduling the next
> thread-in-waiting, which is achieved by your patches. 
No, sorry, I should have been clearer.  I meant that going to the effort
of not increasing the lock size to record "in slowpath" state.

    J

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