Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 0/8] [PATCH RFC V2] Paravirtualized ticketlocks
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
[ Changes since last posting:
- fold all the cleanup/bugfix patches into their base patches
- change spin_lock to make sure fastpath has no cruft in it
- make sure it doesn''t attempt to inline unlock
]
NOTE: this series is based on tip.git tip/x86/spinlocks
This series replaces the existing paravirtualized spinlock mechanism
with a paravirtualized ticketlock mechanism.
Ticket locks have an inherent problem in a virtualized case, because
the vCPUs are scheduled rather than running concurrently (ignoring
gang scheduled vCPUs). This can result in catastrophic performance
collapses when the vCPU scheduler doesn''t schedule the correct
"next"
vCPU, and ends up scheduling a vCPU which burns its entire timeslice
spinning. (Note that this is not the same problem as lock-holder
preemption, which this series also addresses; that''s also a problem,
but not catastrophic).
(See Thomas Friebel''s talk "Prevent Guests from Spinning
Around"
http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.)
Currently we deal with this by having PV spinlocks, which adds a layer
of indirection in front of all the spinlock functions, and defining a
completely new implementation for Xen (and for other pvops users, but
there are none at present).
PV ticketlocks keeps the existing ticketlock implemenentation
(fastpath) as-is, but adds a couple of pvops for the slow paths:
- If a CPU has been waiting for a spinlock for SPIN_THRESHOLD
iterations, then call out to the __ticket_lock_spinning() pvop,
which allows a backend to block the vCPU rather than spinning. This
pvop can set the lock into "slowpath state".
- When releasing a lock, if it is in "slowpath state", the call
__ticket_unlock_kick() to kick the next vCPU in line awake. If the
lock is no longer in contention, it also clears the slowpath flag.
The "slowpath state" is stored in the LSB of the within the lock
ticket. This has the effect of reducing the max number of CPUs by
half (so, a "small ticket" can deal with 128 CPUs, and "large
ticket"
32768).
This series provides a Xen implementation, but it should be
straightforward to add a KVM implementation as well.
Overall, it results in a large reduction in code, it makes the native
and virtualized cases closer, and it removes a layer of indirection
around all the spinlock functions.
The fast path (taking an uncontended lock which isn''t in
"slowpath"
state) is optimal, identical to the non-paravirtualized case.
The inner part of ticket lock code becomes:
inc = xadd(&lock->tickets, inc);
inc.tail &= ~TICKET_SLOWPATH_FLAG;
if (likely(inc.head == inc.tail))
goto out;
for (;;) {
unsigned count = SPIN_THRESHOLD;
do {
if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
goto out;
cpu_relax();
} while (--count);
__ticket_lock_spinning(lock, inc.tail);
}
out: barrier();
which results in:
push %rbp
mov %rsp,%rbp
mov $0x200,%eax
lock xadd %ax,(%rdi)
movzbl %ah,%edx
cmp %al,%dl
jne 1f
pop %rbp
retq
### SLOWPATH START
1: and $-2,%edx
movzbl %dl,%esi
2: mov $0x800,%eax
jmp 4f
3: pause
sub $0x1,%eax
je 5f
4: movzbl (%rdi),%ecx
cmp %cl,%dl
jne 3b
pop %rbp
retq
5: callq *__ticket_lock_spinning
jmp 2b
### SLOWPATH END
with CONFIG_PARAVIRT_SPINLOCKS=n, the code has changed slightly, where
the fastpath case is straight through (taking the lock without
contention), and the spin loop is out of line:
push %rbp
mov %rsp,%rbp
mov $0x100,%eax
lock xadd %ax,(%rdi)
movzbl %ah,%edx
cmp %al,%dl
jne 1f
pop %rbp
retq
### SLOWPATH START
1: pause
movzbl (%rdi),%eax
cmp %dl,%al
jne 1b
pop %rbp
retq
### SLOWPATH END
The unlock code is very straightforward:
__ticket_unlock_release(lock);
if (unlikely(__ticket_in_slowpath(lock)))
__ticket_unlock_slowpath(lock);
which generates:
push %rbp
mov %rsp,%rbp
addb $0x2,(%rdi)
testb $0x1,0x1(%rdi)
jne 1f
pop %rbp
retq
### SLOWPATH START
1: movzwl (%rdi),%edx
movzbl %dh,%ecx
mov %edx,%eax
and $-2,%ecx # clear TICKET_SLOWPATH_FLAG
mov %cl,%dh
cmp %dl,%cl # test to see if lock is uncontended
je 3f
2: movzbl %dl,%esi
callq *__ticket_unlock_kick # kick anyone waiting
pop %rbp
retq
3: lock cmpxchg %dx,(%rdi) # use cmpxchg to safely write back flag
jmp 2b
### SLOWPATH END
The fastpath is pretty straightforward, but it is definitely more
complex than a simple "incb (%rdi)" - which is still generated (and
inlined) when PARAVIRT_SPINLOCKS is disabled.
Thoughts? Comments? Suggestions?
Thanks,
J
Jeremy Fitzhardinge (8):
x86/spinlocks: replace pv spinlocks with pv ticketlocks
x86/ticketlock: don''t inline _spin_unlock when using paravirt
spinlocks
x86/ticketlock: collapse a layer of functions
xen/pvticketlock: Xen implementation for PV ticket locks
x86/pvticketlock: use callee-save for lock_spinning
x86/ticketlocks: when paravirtualizing ticket locks, increment by 2
x86/ticketlock: add slowpath logic
xen/pvticketlock: allow interrupts to be enabled while blocking
arch/x86/Kconfig | 3 +
arch/x86/include/asm/paravirt.h | 30 +---
arch/x86/include/asm/paravirt_types.h | 10 +-
arch/x86/include/asm/spinlock.h | 134 ++++++++++-----
arch/x86/include/asm/spinlock_types.h | 16 ++-
arch/x86/kernel/paravirt-spinlocks.c | 16 +--
arch/x86/xen/spinlock.c | 311 ++++++++-------------------------
kernel/Kconfig.locks | 2 +-
8 files changed, 193 insertions(+), 329 deletions(-)
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 1/8] x86/spinlocks: replace pv spinlocks with pv ticketlocks
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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@citrix.com>
---
arch/x86/include/asm/paravirt.h | 30 ++--------------
arch/x86/include/asm/paravirt_types.h | 10 ++---
arch/x86/include/asm/spinlock.h | 59 ++++++++++++++++++++++++++-------
arch/x86/include/asm/spinlock_types.h | 4 --
arch/x86/kernel/paravirt-spinlocks.c | 15 +-------
arch/x86/xen/spinlock.c | 7 +++-
6 files changed, 63 insertions(+), 62 deletions(-)
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index a7d2db9..76cae7a 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -750,36 +750,14 @@ 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 __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
__ticket_t 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 __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t 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 8e8b9a4..005e24d 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -327,13 +327,11 @@ struct pv_mmu_ops {
};
struct arch_spinlock;
+#include <asm/spinlock_types.h>
+
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, __ticket_t ticket);
+ void (*unlock_kick)(struct arch_spinlock *lock, __ticket_t 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 f5695ee..bafed3b 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -37,6 +37,32 @@
# define UNLOCK_LOCK_PREFIX
#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,
__ticket_t ticket)
+{
+}
+
+static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
+{
+}
+
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+
+
+/*
+ * 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);
+}
+
/*
* 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
@@ -54,19 +80,24 @@
* 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.
*/
-static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
+static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
{
register struct __raw_tickets inc = { .tail = 1 };
inc = xadd(&lock->tickets, inc);
for (;;) {
- if (inc.head == inc.tail)
- break;
- 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);
}
- barrier(); /* make sure nothing creeps before the lock is taken */
+out: barrier(); /* make sure nothing creeps before the lock is taken */
}
static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
@@ -84,7 +115,7 @@ static __always_inline int
__ticket_spin_trylock(arch_spinlock_t *lock)
}
#if (NR_CPUS < 256)
-static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
+static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
{
asm volatile(UNLOCK_LOCK_PREFIX "incb %0"
: "+m" (lock->head_tail)
@@ -92,7 +123,7 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
: "memory", "cc");
}
#else
-static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
+static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
{
asm volatile(UNLOCK_LOCK_PREFIX "incw %0"
: "+m" (lock->head_tail)
@@ -101,6 +132,14 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
}
#endif
+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);
+}
+
static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
{
struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
@@ -115,8 +154,6 @@ 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);
@@ -149,8 +186,6 @@ 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/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index 8ebd5df..dbe223d 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -1,10 +1,6 @@
#ifndef _ASM_X86_SPINLOCK_TYPES_H
#define _ASM_X86_SPINLOCK_TYPES_H
-#ifndef __LINUX_SPINLOCK_TYPES_H
-# error "please don''t include this file directly"
-#endif
-
#include <linux/types.h>
#if (CONFIG_NR_CPUS < 256)
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 cc9b1e1..23af06a 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
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 2/8] x86/ticketlock: don''t inline _spin_unlock when using paravirt spinlocks
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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@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 6a47bb2..1f03f82 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -585,6 +585,9 @@ 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 5068e2a..584637b 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 -- 1.7.6 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 3/8] x86/ticketlock: collapse a layer of functions
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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@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 bafed3b..d1a3970 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -80,7 +80,7 @@ static __always_inline void __ticket_unlock_kick(struct
arch_spinlock *lock, __t
* 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.
*/
-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 = { .tail = 1 };
@@ -100,7 +100,7 @@ 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;
@@ -132,7 +132,7 @@ static __always_inline void
__ticket_unlock_release(arch_spinlock_t *lock)
}
#endif
-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;
@@ -140,46 +140,21 @@ static __always_inline void
__ticket_spin_unlock(arch_spinlock_t *lock)
__ticket_unlock_kick(lock, next);
}
-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)
{
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 4/8] xen/pvticketlock: Xen implementation for PV ticket locks
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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.
We need to make sure interrupts are disabled while we''re relying on the
contents of the per-cpu lock_waiting values, otherwise an interrupt
handler could come in, try to take some other lock, block, and overwrite
our values.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/xen/spinlock.c | 287 +++++++----------------------------------------
1 files changed, 43 insertions(+), 244 deletions(-)
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 23af06a..f6133c5 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 +78,84 @@ 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)
+static void xen_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
{
- 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 = __this_cpu_read(lock_spinners);
- __this_cpu_write(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 */
- __this_cpu_write(lock_spinners, prev);
-}
-
-static noinline int xen_spin_lock_slow(struct arch_spinlock *lock, bool
irq_enable)
-{
- struct xen_spinlock *xl = (struct xen_spinlock *)lock;
- struct xen_spinlock *prev;
int irq = __this_cpu_read(lock_kicker_irq);
- int ret;
+ struct xen_lock_waiting *w = &__get_cpu_var(lock_waiting);
+ int cpu = smp_processor_id();
u64 start;
+ unsigned long flags;
/* 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);
+ /* Make sure interrupts are disabled to ensure that these
+ per-cpu values are not overwritten. */
+ local_irq_save(flags);
+
+ 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);
- 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;
+ cpumask_clear_cpu(cpu, &waiting_cpus);
+ w->lock = NULL;
- 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");
+ local_irq_restore(flags);
- 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));
+ spin_time_accum_blocked(start);
}
-static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
+static void xen_unlock_kick(struct arch_spinlock *lock, __ticket_t 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 +163,6 @@ 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 +197,8 @@ 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 +216,21 @@ 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);
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 5/8] x86/pvticketlock: use callee-save for lock_spinning
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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@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 76cae7a..50281c7 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -752,7 +752,7 @@ static inline void __set_fixmap(unsigned /* enum
fixed_addresses */ idx,
static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
__ticket_t ticket)
{
- PVOP_VCALL2(pv_lock_ops.lock_spinning, lock, ticket);
+ PVOP_VCALLEE2(pv_lock_ops.lock_spinning, lock, ticket);
}
static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
index 005e24d..5e0c138 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -330,7 +330,7 @@ struct arch_spinlock;
#include <asm/spinlock_types.h>
struct pv_lock_ops {
- void (*lock_spinning)(struct arch_spinlock *lock, __ticket_t ticket);
+ struct paravirt_callee_save lock_spinning;
void (*unlock_kick)(struct arch_spinlock *lock, __ticket_t 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 f6133c5..7a04950 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -145,6 +145,7 @@ out:
spin_time_accum_blocked(start);
}
+PV_CALLEE_SAVE_REGS_THUNK(xen_lock_spinning);
static void xen_unlock_kick(struct arch_spinlock *lock, __ticket_t next)
{
@@ -197,7 +198,7 @@ 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;
}
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 6/8] x86/ticketlocks: when paravirtualizing ticket locks, increment by 2
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@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@citrix.com>
---
arch/x86/include/asm/spinlock.h | 16 ++++++++--------
arch/x86/include/asm/spinlock_types.h | 10 +++++++++-
2 files changed, 17 insertions(+), 9 deletions(-)
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index d1a3970..7a1c0c4 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -82,7 +82,7 @@ static __always_inline void __ticket_unlock_kick(struct
arch_spinlock *lock, __t
*/
static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
{
- register struct __raw_tickets inc = { .tail = 1 };
+ register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
inc = xadd(&lock->tickets, inc);
@@ -108,7 +108,7 @@ 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;
@@ -117,24 +117,24 @@ static __always_inline int
arch_spin_trylock(arch_spinlock_t *lock)
#if (NR_CPUS < 256)
static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
{
- asm volatile(UNLOCK_LOCK_PREFIX "incb %0"
+ asm volatile(UNLOCK_LOCK_PREFIX "addb %1, %0"
: "+m" (lock->head_tail)
- :
+ : "i" (TICKET_LOCK_INC)
: "memory", "cc");
}
#else
static __always_inline void __ticket_unlock_release(arch_spinlock_t *lock)
{
- asm volatile(UNLOCK_LOCK_PREFIX "incw %0"
+ asm volatile(UNLOCK_LOCK_PREFIX "addw %1, %0"
: "+m" (lock->head_tail)
- :
+ : "i" (TICKET_LOCK_INC)
: "memory", "cc");
}
#endif
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);
@@ -151,7 +151,7 @@ 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 dbe223d..aa9a205 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -3,7 +3,13 @@
#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
@@ -11,6 +17,8 @@ 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))
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 7/8] x86/ticketlock: add slowpath logic
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Maintain a flag in the LSB of the ticket lock tail 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 (ie,
no contention).
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
unlock
test slowpath
-> false
set slowpath flags
block
Whereas this works in any ordering:
Unlocker Locker
set slowpath flags
test for lock pickup
-> fail
block
unlock
test slowpath
-> true, kick
Note: this code relies on gcc making sure that unlikely() code is out of
line of the fastpath, which only happens when OPTIMIZE_SIZE=n. If it
doesn''t the generated code isn''t too bad, but its definitely
suboptimal.
(Thanks to Srivatsa Vaddagiri for providing a bugfix to the original
version of this change, which has been folded in.)
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
---
arch/x86/include/asm/paravirt.h | 2 +-
arch/x86/include/asm/spinlock.h | 72 ++++++++++++++++++++++++++-------
arch/x86/include/asm/spinlock_types.h | 2 +
arch/x86/kernel/paravirt-spinlocks.c | 1 +
arch/x86/xen/spinlock.c | 4 ++
5 files changed, 65 insertions(+), 16 deletions(-)
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 50281c7..13b3d8b 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -755,7 +755,7 @@ static __always_inline void __ticket_lock_spinning(struct
arch_spinlock *lock, _
PVOP_VCALLEE2(pv_lock_ops.lock_spinning, lock, ticket);
}
-static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
+static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t 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 7a1c0c4..64422f1 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -40,29 +40,46 @@
/* How long a lock should spin before we consider blocking */
#define SPIN_THRESHOLD (1 << 11)
-#ifndef CONFIG_PARAVIRT_SPINLOCKS
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
-static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock,
__ticket_t ticket)
+/*
+ * 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 __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock,
__ticket_t ticket)
+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");
}
-#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+#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,
__ticket_t ticket)
+{
+}
-/*
- * 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)
+static inline void __ticket_unlock_kick(struct arch_spinlock *lock, __ticket_t
ticket)
{
- if (unlikely(lock->tickets.tail != next))
- ____ticket_unlock_kick(lock, next);
}
+#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
@@ -85,15 +102,17 @@ static __always_inline void arch_spin_lock(struct
arch_spinlock *lock)
register struct __raw_tickets inc = { .tail = TICKET_LOCK_INC };
inc = xadd(&lock->tickets, inc);
+ if (likely(inc.head == inc.tail))
+ goto out;
+ inc.tail &= ~TICKET_SLOWPATH_FLAG;
for (;;) {
unsigned count = SPIN_THRESHOLD;
do {
- if (inc.head == inc.tail)
+ if (ACCESS_ONCE(lock->tickets.head) == inc.tail)
goto out;
cpu_relax();
- inc.head = ACCESS_ONCE(lock->tickets.head);
} while (--count);
__ticket_lock_spinning(lock, inc.tail);
}
@@ -132,12 +151,35 @@ static __always_inline void
__ticket_unlock_release(arch_spinlock_t *lock)
}
#endif
+static inline void __ticket_unlock_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;
+
+ /* Clear the slowpath flag */
+ new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+
+ /*
+ * If the lock is uncontended, clear the flag - use cmpxchg in
+ * case it changes behind our back though.
+ */
+ if (new.tickets.head == new.tickets.tail)
+ cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
+
+ /* Wake up an appropriate waiter */
+ __ticket_unlock_kick(lock, new.tickets.head);
+}
+
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);
+ if (unlikely(__ticket_in_slowpath(lock)))
+ __ticket_unlock_slowpath(lock);
}
static inline int arch_spin_is_locked(arch_spinlock_t *lock)
diff --git a/arch/x86/include/asm/spinlock_types.h
b/arch/x86/include/asm/spinlock_types.h
index aa9a205..407f7f7 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -5,8 +5,10 @@
#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..0883c48 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -15,3 +15,4 @@ struct pv_lock_ops pv_lock_ops = {
};
EXPORT_SYMBOL(pv_lock_ops);
+
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 7a04950..c939723 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -124,6 +124,10 @@ static void xen_lock_spinning(struct arch_spinlock *lock,
__ticket_t 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) {
--
1.7.6
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Jeremy Fitzhardinge
2011-Sep-02 23:54 UTC
[Xen-devel] [PATCH 8/8] xen/pvticketlock: allow interrupts to be enabled while blocking
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
If interrupts were enabled when taking the spinlock, we can leave them
enabled while blocking to get the lock.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/xen/spinlock.c | 42 +++++++++++++++++++++++++++++++++++-------
1 files changed, 35 insertions(+), 7 deletions(-)
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index c939723..d2335f88 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -106,11 +106,28 @@ static void xen_lock_spinning(struct arch_spinlock *lock,
__ticket_t want)
start = spin_time_start();
- /* Make sure interrupts are disabled to ensure that these
- per-cpu values are not overwritten. */
+ /*
+ * Make sure an interrupt handler can''t upset things in a
+ * partially setup state.
+ */
local_irq_save(flags);
+ /*
+ * We don''t really care if we''re overwriting some other
+ * (lock,want) pair, as that would mean that we''re currently
+ * in an interrupt context, and the outer context had
+ * interrupts enabled. That has already kicked the VCPU out
+ * of xen_poll_irq(), so it will just return spuriously and
+ * retry with newly setup (lock,want).
+ *
+ * The ordering protocol on this is that the "lock" pointer
+ * may only be set non-NULL if the "want" ticket is correct.
+ * If we''re updating "want", we must first clear
"lock".
+ */
+ w->lock = NULL;
+ smp_wmb();
w->want = want;
+ smp_wmb();
w->lock = lock;
/* This uses set_bit, which atomic and therefore a barrier */
@@ -124,21 +141,30 @@ static void xen_lock_spinning(struct arch_spinlock *lock,
__ticket_t 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. */
+ /*
+ * 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 */
+ /*
+ * 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;
}
+ /* Allow interrupts while blocked */
+ local_irq_restore(flags);
+
/* Block until irq becomes pending (or perhaps a spurious wakeup) */
xen_poll_irq(irq);
ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
+ local_irq_save(flags);
+
kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));
out:
@@ -160,7 +186,9 @@ static void xen_unlock_kick(struct arch_spinlock *lock,
__ticket_t next)
for_each_cpu(cpu, &waiting_cpus) {
const struct xen_lock_waiting *w = &per_cpu(lock_waiting, cpu);
- if (w->lock == lock && w->want == next) {
+ /* Make sure we read lock before want */
+ if (ACCESS_ONCE(w->lock) == lock &&
+ ACCESS_ONCE(w->want) == next) {
ADD_STATS(released_slow_kicked, 1);
xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
break;
--
1.7.6
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