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Pan Xinhui

2016-Nov-02 09:08 UTC

[PATCH v7 00/11] implement vcpu preempted check

change from v6:
	fix typos and remove uncessary comments.
change from v5:
	spilt x86/kvm patch into guest/host part.
	introduce kvm_write_guest_offset_cached.
	fix some typos.
	rebase patch onto 4.9.2
change from v4:
	spilt x86 kvm vcpu preempted check into two patches.
	add documentation patch.
	add x86 vcpu preempted check patch under xen
	add s390 vcpu preempted check patch 
change from v3:
	add x86 vcpu preempted check patch
change from v2:
	no code change, fix typos, update some comments
change from v1:
	a simplier definition of default vcpu_is_preempted
	skip mahcine type check on ppc, and add config. remove dedicated macro.
	add one patch to drop overload of rwsem_spin_on_owner and mutex_spin_on_owner. 
	add more comments
	thanks boqun and Peter's suggestion.

This patch set aims to fix lock holder preemption issues.

test-case:
perf record -a perf bench sched messaging -g 400 -p && perf report

18.09%  sched-messaging  [kernel.vmlinux]  [k] osq_lock
12.28%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
 5.27%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 3.89%  sched-messaging  [kernel.vmlinux]  [k] wait_consider_task
 3.64%  sched-messaging  [kernel.vmlinux]  [k] _raw_write_lock_irq
 3.41%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner.is
 2.49%  sched-messaging  [kernel.vmlinux]  [k] system_call

We introduce interface bool vcpu_is_preempted(int cpu) and use it in some spin
loops of osq_lock, rwsem_spin_on_owner and mutex_spin_on_owner.
These spin_on_onwer variant also cause rcu stall before we apply this patch set

We also have observed some performace improvements in uninx benchmark tests.

PPC test result:
1 copy - 0.94%
2 copy - 7.17%
4 copy - 11.9%
8 copy -  3.04%
16 copy - 15.11%

details below:
Without patch:

1 copy - File Write 4096 bufsize 8000 maxblocks      2188223.0 KBps  (30.0 s, 1
samples)
2 copy - File Write 4096 bufsize 8000 maxblocks      1804433.0 KBps  (30.0 s, 1
samples)
4 copy - File Write 4096 bufsize 8000 maxblocks      1237257.0 KBps  (30.0 s, 1
samples)
8 copy - File Write 4096 bufsize 8000 maxblocks      1032658.0 KBps  (30.0 s, 1
samples)
16 copy - File Write 4096 bufsize 8000 maxblocks       768000.0 KBps  (30.1 s, 1
samples)

With patch: 

1 copy - File Write 4096 bufsize 8000 maxblocks      2209189.0 KBps  (30.0 s, 1
samples)
2 copy - File Write 4096 bufsize 8000 maxblocks      1943816.0 KBps  (30.0 s, 1
samples)
4 copy - File Write 4096 bufsize 8000 maxblocks      1405591.0 KBps  (30.0 s, 1
samples)
8 copy - File Write 4096 bufsize 8000 maxblocks      1065080.0 KBps  (30.0 s, 1
samples)
16 copy - File Write 4096 bufsize 8000 maxblocks       904762.0 KBps  (30.0 s, 1
samples)

X86 test result:
	test-case			after-patch	  before-patch
Execl Throughput                       |    18307.9 lps  |    11701.6 lps 
File Copy 1024 bufsize 2000 maxblocks  |  1352407.3 KBps |   790418.9 KBps
File Copy 256 bufsize 500 maxblocks    |   367555.6 KBps |   222867.7 KBps
File Copy 4096 bufsize 8000 maxblocks  |  3675649.7 KBps |  1780614.4 KBps
Pipe Throughput                        | 11872208.7 lps  | 11855628.9 lps 
Pipe-based Context Switching           |  1495126.5 lps  |  1490533.9 lps 
Process Creation                       |    29881.2 lps  |    28572.8 lps 
Shell Scripts (1 concurrent)           |    23224.3 lpm  |    22607.4 lpm 
Shell Scripts (8 concurrent)           |     3531.4 lpm  |     3211.9 lpm 
System Call Overhead                   | 10385653.0 lps  | 10419979.0 lps 


Christian Borntraeger (1):
  s390/spinlock: Provide vcpu_is_preempted

Juergen Gross (1):
  x86, xen: support vcpu preempted check

Pan Xinhui (9):
  kernel/sched: introduce vcpu preempted check interface
  locking/osq: Drop the overload of osq_lock()
  kernel/locking: Drop the overload of {mutex,rwsem}_spin_on_owner
  powerpc/spinlock: support vcpu preempted check
  x86, paravirt: Add interface to support kvm/xen vcpu preempted check
  KVM: Introduce kvm_write_guest_offset_cached
  x86, kvm/x86.c: support vcpu preempted check
  x86, kernel/kvm.c: support vcpu preempted check
  Documentation: virtual: kvm: Support vcpu preempted check

 Documentation/virtual/kvm/msr.txt     |  9 ++++++++-
 arch/powerpc/include/asm/spinlock.h   |  8 ++++++++
 arch/s390/include/asm/spinlock.h      |  8 ++++++++
 arch/s390/kernel/smp.c                |  9 +++++++--
 arch/s390/lib/spinlock.c              | 25 ++++++++-----------------
 arch/x86/include/asm/paravirt_types.h |  2 ++
 arch/x86/include/asm/spinlock.h       |  8 ++++++++
 arch/x86/include/uapi/asm/kvm_para.h  |  4 +++-
 arch/x86/kernel/kvm.c                 | 12 ++++++++++++
 arch/x86/kernel/paravirt-spinlocks.c  |  6 ++++++
 arch/x86/kvm/x86.c                    | 16 ++++++++++++++++
 arch/x86/xen/spinlock.c               |  3 ++-
 include/linux/kvm_host.h              |  2 ++
 include/linux/sched.h                 | 12 ++++++++++++
 kernel/locking/mutex.c                | 13 +++++++++++--
 kernel/locking/osq_lock.c             |  8 +++++++-
 kernel/locking/rwsem-xadd.c           | 14 +++++++++++---
 virt/kvm/kvm_main.c                   | 20 ++++++++++++++------
 18 files changed, 145 insertions(+), 34 deletions(-)

-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 01/11] kernel/sched: introduce vcpu preempted check interface

This patch support to fix lock holder preemption issue.

For kernel users, we could use bool vcpu_is_preempted(int cpu) to detect
if one vcpu is preempted or not.

The default implementation is a macro defined by false. So compiler can
wrap it out if arch dose not support such vcpu preempted check.

Suggested-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger at de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
Tested-by: Juergen Gross <jgross at suse.com>
---
 include/linux/sched.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 348f51b..44c1ce7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -3506,6 +3506,18 @@ static inline void set_task_cpu(struct task_struct *p,
unsigned int cpu)
 
 #endif /* CONFIG_SMP */
 
+/*
+ * In order to deal with a various lock holder preemption issues provide an
+ * interface to see if a vCPU is currently running or not.
+ *
+ * This allows us to terminate optimistic spin loops and block, analogous to
+ * the native optimistic spin heuristic of testing if the lock owner task is
+ * running or not.
+ */
+#ifndef vcpu_is_preempted
+#define vcpu_is_preempted(cpu)	false
+#endif
+
 extern long sched_setaffinity(pid_t pid, const struct cpumask *new_mask);
 extern long sched_getaffinity(pid_t pid, struct cpumask *mask);
 
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 02/11] locking/osq: Drop the overload of osq_lock()

An over-committed guest with more vCPUs than pCPUs has a heavy overload
in osq_lock().

This is because vCPU A hold the osq lock and yield out, vCPU B wait
per_cpu node->locked to be set. IOW, vCPU B wait vCPU A to run and
unlock the osq lock.

Kernel has an interface bool vcpu_is_preempted(int cpu) to detect if a
vCPU is currently running or not. So break the spin loops on true
condition.

test case:
perf record -a perf bench sched messaging -g 400 -p && perf report

before patch:
18.09%  sched-messaging  [kernel.vmlinux]  [k] osq_lock
12.28%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
 5.27%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 3.89%  sched-messaging  [kernel.vmlinux]  [k] wait_consider_task
 3.64%  sched-messaging  [kernel.vmlinux]  [k] _raw_write_lock_irq
 3.41%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner.is
 2.49%  sched-messaging  [kernel.vmlinux]  [k] system_call

after patch:
20.68%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner
 8.45%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 4.12%  sched-messaging  [kernel.vmlinux]  [k] system_call
 3.01%  sched-messaging  [kernel.vmlinux]  [k] system_call_common
 2.83%  sched-messaging  [kernel.vmlinux]  [k] copypage_power7
 2.64%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
 2.00%  sched-messaging  [kernel.vmlinux]  [k] osq_lock

Suggested-by: Boqun Feng <boqun.feng at gmail.com>
Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger at de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
Tested-by: Juergen Gross <jgross at suse.com>
---
 kernel/locking/osq_lock.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..091f97f 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -21,6 +21,11 @@ static inline int encode_cpu(int cpu_nr)
 	return cpu_nr + 1;
 }
 
+static inline int node_cpu(struct optimistic_spin_node *node)
+{
+	return node->cpu - 1;
+}
+
 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
 {
 	int cpu_nr = encoded_cpu_val - 1;
@@ -118,8 +123,9 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	while (!READ_ONCE(node->locked)) {
 		/*
 		 * If we need to reschedule bail... so we can block.
+		 * Use vcpu_is_preempted to detect lock holder preemption issue.
 		 */
-		if (need_resched())
+		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
 			goto unqueue;
 
 		cpu_relax_lowlatency();
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 03/11] kernel/locking: Drop the overload of {mutex, rwsem}_spin_on_owner

An over-committed guest with more vCPUs than pCPUs has a heavy overload
in the two spin_on_owner. This blames on the lock holder preemption
issue.

Kernel has an interface bool vcpu_is_preempted(int cpu) to see if a vCPU
is currently running or not. So break the spin loops on true condition.

test-case:
perf record -a perf bench sched messaging -g 400 -p && perf report

before patch:
20.68%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner
 8.45%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 4.12%  sched-messaging  [kernel.vmlinux]  [k] system_call
 3.01%  sched-messaging  [kernel.vmlinux]  [k] system_call_common
 2.83%  sched-messaging  [kernel.vmlinux]  [k] copypage_power7
 2.64%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
 2.00%  sched-messaging  [kernel.vmlinux]  [k] osq_lock

after patch:
 9.99%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 5.28%  sched-messaging  [unknown]         [H] 0xc0000000000768e0
 4.27%  sched-messaging  [kernel.vmlinux]  [k] __copy_tofrom_user_power7
 3.77%  sched-messaging  [kernel.vmlinux]  [k] copypage_power7
 3.24%  sched-messaging  [kernel.vmlinux]  [k] _raw_write_lock_irq
 3.02%  sched-messaging  [kernel.vmlinux]  [k] system_call
 2.69%  sched-messaging  [kernel.vmlinux]  [k] wait_consider_task

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger at de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
Tested-by: Juergen Gross <jgross at suse.com>
---
 kernel/locking/mutex.c      | 13 +++++++++++--
 kernel/locking/rwsem-xadd.c | 14 +++++++++++---
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a70b90d..24face6 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -236,7 +236,11 @@ bool mutex_spin_on_owner(struct mutex *lock, struct
task_struct *owner)
 		 */
 		barrier();
 
-		if (!owner->on_cpu || need_resched()) {
+		/*
+		 * Use vcpu_is_preempted to detect lock holder preemption issue.
+		 */
+		if (!owner->on_cpu || need_resched() ||
+				vcpu_is_preempted(task_cpu(owner))) {
 			ret = false;
 			break;
 		}
@@ -261,8 +265,13 @@ static inline int mutex_can_spin_on_owner(struct mutex
*lock)
 
 	rcu_read_lock();
 	owner = READ_ONCE(lock->owner);
+
+	/*
+	 * As lock holder preemption issue, we both skip spinning if task is not
+	 * on cpu or its cpu is preempted
+	 */
 	if (owner)
-		retval = owner->on_cpu;
+		retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
 	rcu_read_unlock();
 	/*
 	 * if lock->owner is not set, the mutex owner may have just acquired
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2337b4b..b664ce1 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -336,7 +336,11 @@ static inline bool rwsem_can_spin_on_owner(struct
rw_semaphore *sem)
 		goto done;
 	}
 
-	ret = owner->on_cpu;
+	/*
+	 * As lock holder preemption issue, we both skip spinning if task is not
+	 * on cpu or its cpu is preempted
+	 */
+	ret = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
 done:
 	rcu_read_unlock();
 	return ret;
@@ -362,8 +366,12 @@ static noinline bool rwsem_spin_on_owner(struct
rw_semaphore *sem)
 		 */
 		barrier();
 
-		/* abort spinning when need_resched or owner is not running */
-		if (!owner->on_cpu || need_resched()) {
+		/*
+		 * abort spinning when need_resched or owner is not running or
+		 * owner's cpu is preempted.
+		 */
+		if (!owner->on_cpu || need_resched() ||
+				vcpu_is_preempted(task_cpu(owner))) {
 			rcu_read_unlock();
 			return false;
 		}
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 04/11] powerpc/spinlock: support vcpu preempted check

This is to fix some lock holder preemption issues. Some other locks
implementation do a spin loop before acquiring the lock itself.
Currently kernel has an interface of bool vcpu_is_preempted(int cpu). It
takes the cpu as parameter and return true if the cpu is preempted. Then
kernel can break the spin loops upon the retval of vcpu_is_preempted.

As kernel has used this interface, So lets support it.

Only pSeries need support it. And the fact is PowerNV is built into same
kernel image with pSeries. So we need return false if we are runnig as
PowerNV. The another fact is that lppaca->yiled_count keeps zero on
PowerNV. So we can just skip the machine type check.

Suggested-by: Boqun Feng <boqun.feng at gmail.com>
Suggested-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/spinlock.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/include/asm/spinlock.h
b/arch/powerpc/include/asm/spinlock.h
index fa37fe9..8c1b913 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h
@@ -52,6 +52,14 @@
 #define SYNC_IO
 #endif
 
+#ifdef CONFIG_PPC_PSERIES
+#define vcpu_is_preempted vcpu_is_preempted
+static inline bool vcpu_is_preempted(int cpu)
+{
+	return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1);
+}
+#endif
+
 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 {
 	return lock.slock == 0;
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 05/11] s390/spinlock: Provide vcpu_is_preempted

From: Christian Borntraeger <borntraeger at de.ibm.com>

this implements the s390 backend for commit
"kernel/sched: introduce vcpu preempted check interface"
by reworking the existing smp_vcpu_scheduled into
arch_vcpu_is_preempted. We can then also get rid of the
local cpu_is_preempted function by moving the
CIF_ENABLED_WAIT test into arch_vcpu_is_preempted.

Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens at de.ibm.com>
---
 arch/s390/include/asm/spinlock.h |  8 ++++++++
 arch/s390/kernel/smp.c           |  9 +++++++--
 arch/s390/lib/spinlock.c         | 25 ++++++++-----------------
 3 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/arch/s390/include/asm/spinlock.h b/arch/s390/include/asm/spinlock.h
index 7e9e09f..7ecd890 100644
--- a/arch/s390/include/asm/spinlock.h
+++ b/arch/s390/include/asm/spinlock.h
@@ -23,6 +23,14 @@ _raw_compare_and_swap(unsigned int *lock, unsigned int old,
unsigned int new)
 	return __sync_bool_compare_and_swap(lock, old, new);
 }
 
+#ifndef CONFIG_SMP
+static inline bool arch_vcpu_is_preempted(int cpu) { return false; }
+#else
+bool arch_vcpu_is_preempted(int cpu);
+#endif
+
+#define vcpu_is_preempted arch_vcpu_is_preempted
+
 /*
  * Simple spin lock operations.  There are two variants, one clears IRQ's
  * on the local processor, one does not.
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 35531fe..b988ed1 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -368,10 +368,15 @@ int smp_find_processor_id(u16 address)
 	return -1;
 }
 
-int smp_vcpu_scheduled(int cpu)
+bool arch_vcpu_is_preempted(int cpu)
 {
-	return pcpu_running(pcpu_devices + cpu);
+	if (test_cpu_flag_of(CIF_ENABLED_WAIT, cpu))
+		return false;
+	if (pcpu_running(pcpu_devices + cpu))
+		return false;
+	return true;
 }
+EXPORT_SYMBOL(arch_vcpu_is_preempted);
 
 void smp_yield_cpu(int cpu)
 {
diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
index e5f50a7..e48a48e 100644
--- a/arch/s390/lib/spinlock.c
+++ b/arch/s390/lib/spinlock.c
@@ -37,15 +37,6 @@ static inline void _raw_compare_and_delay(unsigned int *lock,
unsigned int old)
 	asm(".insn rsy,0xeb0000000022,%0,0,%1" : : "d" (old),
"Q" (*lock));
 }
 
-static inline int cpu_is_preempted(int cpu)
-{
-	if (test_cpu_flag_of(CIF_ENABLED_WAIT, cpu))
-		return 0;
-	if (smp_vcpu_scheduled(cpu))
-		return 0;
-	return 1;
-}
-
 void arch_spin_lock_wait(arch_spinlock_t *lp)
 {
 	unsigned int cpu = SPINLOCK_LOCKVAL;
@@ -62,7 +53,7 @@ void arch_spin_lock_wait(arch_spinlock_t *lp)
 			continue;
 		}
 		/* First iteration: check if the lock owner is running. */
-		if (first_diag && cpu_is_preempted(~owner)) {
+		if (first_diag && arch_vcpu_is_preempted(~owner)) {
 			smp_yield_cpu(~owner);
 			first_diag = 0;
 			continue;
@@ -81,7 +72,7 @@ void arch_spin_lock_wait(arch_spinlock_t *lp)
 		 * yield the CPU unconditionally. For LPAR rely on the
 		 * sense running status.
 		 */
-		if (!MACHINE_IS_LPAR || cpu_is_preempted(~owner)) {
+		if (!MACHINE_IS_LPAR || arch_vcpu_is_preempted(~owner)) {
 			smp_yield_cpu(~owner);
 			first_diag = 0;
 		}
@@ -108,7 +99,7 @@ void arch_spin_lock_wait_flags(arch_spinlock_t *lp, unsigned
long flags)
 			continue;
 		}
 		/* Check if the lock owner is running. */
-		if (first_diag && cpu_is_preempted(~owner)) {
+		if (first_diag && arch_vcpu_is_preempted(~owner)) {
 			smp_yield_cpu(~owner);
 			first_diag = 0;
 			continue;
@@ -127,7 +118,7 @@ void arch_spin_lock_wait_flags(arch_spinlock_t *lp, unsigned
long flags)
 		 * yield the CPU unconditionally. For LPAR rely on the
 		 * sense running status.
 		 */
-		if (!MACHINE_IS_LPAR || cpu_is_preempted(~owner)) {
+		if (!MACHINE_IS_LPAR || arch_vcpu_is_preempted(~owner)) {
 			smp_yield_cpu(~owner);
 			first_diag = 0;
 		}
@@ -165,7 +156,7 @@ void _raw_read_lock_wait(arch_rwlock_t *rw)
 	owner = 0;
 	while (1) {
 		if (count-- <= 0) {
-			if (owner && cpu_is_preempted(~owner))
+			if (owner && arch_vcpu_is_preempted(~owner))
 				smp_yield_cpu(~owner);
 			count = spin_retry;
 		}
@@ -211,7 +202,7 @@ void _raw_write_lock_wait(arch_rwlock_t *rw, unsigned int
prev)
 	owner = 0;
 	while (1) {
 		if (count-- <= 0) {
-			if (owner && cpu_is_preempted(~owner))
+			if (owner && arch_vcpu_is_preempted(~owner))
 				smp_yield_cpu(~owner);
 			count = spin_retry;
 		}
@@ -241,7 +232,7 @@ void _raw_write_lock_wait(arch_rwlock_t *rw)
 	owner = 0;
 	while (1) {
 		if (count-- <= 0) {
-			if (owner && cpu_is_preempted(~owner))
+			if (owner && arch_vcpu_is_preempted(~owner))
 				smp_yield_cpu(~owner);
 			count = spin_retry;
 		}
@@ -285,7 +276,7 @@ void arch_lock_relax(unsigned int cpu)
 {
 	if (!cpu)
 		return;
-	if (MACHINE_IS_LPAR && !cpu_is_preempted(~cpu))
+	if (MACHINE_IS_LPAR && !arch_vcpu_is_preempted(~cpu))
 		return;
 	smp_yield_cpu(~cpu);
 }
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check

This is to fix some lock holder preemption issues. Some other locks
implementation do a spin loop before acquiring the lock itself.
Currently kernel has an interface of bool vcpu_is_preempted(int cpu). It
takes the cpu as parameter and return true if the cpu is preempted.
Then kernel can break the spin loops upon the retval of
vcpu_is_preempted.

As kernel has used this interface, So lets support it.

To deal with kernel and kvm/xen, add vcpu_is_preempted into struct
pv_lock_ops.

Then kvm or xen could provide their own implementation to support
vcpu_is_preempted.

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
---
 arch/x86/include/asm/paravirt_types.h | 2 ++
 arch/x86/include/asm/spinlock.h       | 8 ++++++++
 arch/x86/kernel/paravirt-spinlocks.c  | 6 ++++++
 3 files changed, 16 insertions(+)

diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
index 0f400c0..38c3bb7 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -310,6 +310,8 @@ struct pv_lock_ops {
 
 	void (*wait)(u8 *ptr, u8 val);
 	void (*kick)(int cpu);
+
+	bool (*vcpu_is_preempted)(int cpu);
 };
 
 /* 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 921bea7..0526f59 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -26,6 +26,14 @@
 extern struct static_key paravirt_ticketlocks_enabled;
 static __always_inline bool static_key_false(struct static_key *key);
 
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define vcpu_is_preempted vcpu_is_preempted
+static inline bool vcpu_is_preempted(int cpu)
+{
+	return pv_lock_ops.vcpu_is_preempted(cpu);
+}
+#endif
+
 #include <asm/qspinlock.h>
 
 /*
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index 2c55a00..2f204dd 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -21,12 +21,18 @@ bool pv_is_native_spin_unlock(void)
 		__raw_callee_save___native_queued_spin_unlock;
 }
 
+static bool native_vcpu_is_preempted(int cpu)
+{
+	return 0;
+}
+
 struct pv_lock_ops pv_lock_ops = {
 #ifdef CONFIG_SMP
 	.queued_spin_lock_slowpath = native_queued_spin_lock_slowpath,
 	.queued_spin_unlock = PV_CALLEE_SAVE(__native_queued_spin_unlock),
 	.wait = paravirt_nop,
 	.kick = paravirt_nop,
+	.vcpu_is_preempted = native_vcpu_is_preempted,
 #endif /* SMP */
 };
 EXPORT_SYMBOL(pv_lock_ops);
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 07/11] KVM: Introduce kvm_write_guest_offset_cached

It allows us to update some status or field of one struct partially.

We can also save one kvm_read_guest_cached if we just update one filed
of the struct regardless of its current value.

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
---
 include/linux/kvm_host.h |  2 ++
 virt/kvm/kvm_main.c      | 20 ++++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 01c0b9c..6f00237 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -645,6 +645,8 @@ int kvm_write_guest(struct kvm *kvm, gpa_t gpa, const void
*data,
 		    unsigned long len);
 int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 			   void *data, unsigned long len);
+int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache
*ghc,
+			   void *data, int offset, unsigned long len);
 int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 			      gpa_t gpa, unsigned long len);
 int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2907b7b..95308ee 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1972,30 +1972,38 @@ int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct
gfn_to_hva_cache *ghc,
 }
 EXPORT_SYMBOL_GPL(kvm_gfn_to_hva_cache_init);
 
-int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
-			   void *data, unsigned long len)
+int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache
*ghc,
+			   void *data, int offset, unsigned long len)
 {
 	struct kvm_memslots *slots = kvm_memslots(kvm);
 	int r;
+	gpa_t gpa = ghc->gpa + offset;
 
-	BUG_ON(len > ghc->len);
+	BUG_ON(len + offset > ghc->len);
 
 	if (slots->generation != ghc->generation)
 		kvm_gfn_to_hva_cache_init(kvm, ghc, ghc->gpa, ghc->len);
 
 	if (unlikely(!ghc->memslot))
-		return kvm_write_guest(kvm, ghc->gpa, data, len);
+		return kvm_write_guest(kvm, gpa, data, len);
 
 	if (kvm_is_error_hva(ghc->hva))
 		return -EFAULT;
 
-	r = __copy_to_user((void __user *)ghc->hva, data, len);
+	r = __copy_to_user((void __user *)ghc->hva + offset, data, len);
 	if (r)
 		return -EFAULT;
-	mark_page_dirty_in_slot(ghc->memslot, ghc->gpa >> PAGE_SHIFT);
+	mark_page_dirty_in_slot(ghc->memslot, gpa >> PAGE_SHIFT);
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(kvm_write_guest_offset_cached);
+
+int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
+			   void *data, unsigned long len)
+{
+	return kvm_write_guest_offset_cached(kvm, ghc, data, 0, len);
+}
 EXPORT_SYMBOL_GPL(kvm_write_guest_cached);
 
 int kvm_read_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 08/11] x86, kvm/x86.c: support vcpu preempted check

Support the vcpu_is_preempted() functionality under KVM. This will
enhance lock performance on overcommitted hosts (more runnable vcpus
than physical cpus in the system) as doing busy waits for preempted
vcpus will hurt system performance far worse than early yielding.

Use one field of struct kvm_steal_time ::preempted to indicate that if
one vcpu is running or not.

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
---
 arch/x86/include/uapi/asm/kvm_para.h |  4 +++-
 arch/x86/kvm/x86.c                   | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/kvm_para.h
b/arch/x86/include/uapi/asm/kvm_para.h
index 94dc8ca..1421a65 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -45,7 +45,9 @@ struct kvm_steal_time {
 	__u64 steal;
 	__u32 version;
 	__u32 flags;
-	__u32 pad[12];
+	__u8  preempted;
+	__u8  u8_pad[3];
+	__u32 pad[11];
 };
 
 #define KVM_STEAL_ALIGNMENT_BITS 5
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e375235..f06e115 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2057,6 +2057,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
 		return;
 
+	vcpu->arch.st.steal.preempted = 0;
+
 	if (vcpu->arch.st.steal.version & 1)
 		vcpu->arch.st.steal.version += 1;  /* first time write, random junk */
 
@@ -2810,8 +2812,22 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 }
 
+static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
+{
+	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
+		return;
+
+	vcpu->arch.st.steal.preempted = 1;
+
+	kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime,
+			&vcpu->arch.st.steal.preempted,
+			offsetof(struct kvm_steal_time, preempted),
+			sizeof(vcpu->arch.st.steal.preempted));
+}
+
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
+	kvm_steal_time_set_preempted(vcpu);
 	kvm_x86_ops->vcpu_put(vcpu);
 	kvm_put_guest_fpu(vcpu);
 	vcpu->arch.last_host_tsc = rdtsc();
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 09/11] x86, kernel/kvm.c: support vcpu preempted check

Support the vcpu_is_preempted() functionality under KVM. This will
enhance lock performance on overcommitted hosts (more runnable vcpus
than physical cpus in the system) as doing busy waits for preempted
vcpus will hurt system performance far worse than early yielding.

struct kvm_steal_time::preempted indicate that if one vcpu is running or
not after commit("x86, kvm/x86.c: support vcpu preempted check").

unix benchmark result:
host:  kernel 4.8.1, i5-4570, 4 cpus
guest: kernel 4.8.1, 8 vcpus

        test-case                       after-patch       before-patch
Execl Throughput                       |    18307.9 lps  |    11701.6 lps
File Copy 1024 bufsize 2000 maxblocks  |  1352407.3 KBps |   790418.9 KBps
File Copy 256 bufsize 500 maxblocks    |   367555.6 KBps |   222867.7 KBps
File Copy 4096 bufsize 8000 maxblocks  |  3675649.7 KBps |  1780614.4 KBps
Pipe Throughput                        | 11872208.7 lps  | 11855628.9 lps
Pipe-based Context Switching           |  1495126.5 lps  |  1490533.9 lps
Process Creation                       |    29881.2 lps  |    28572.8 lps
Shell Scripts (1 concurrent)           |    23224.3 lpm  |    22607.4 lpm
Shell Scripts (8 concurrent)           |     3531.4 lpm  |     3211.9 lpm
System Call Overhead                   | 10385653.0 lps  | 10419979.0 lps

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
---
 arch/x86/kernel/kvm.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index edbbfc8..0b48dd2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -415,6 +415,15 @@ void kvm_disable_steal_time(void)
 	wrmsr(MSR_KVM_STEAL_TIME, 0, 0);
 }
 
+static bool kvm_vcpu_is_preempted(int cpu)
+{
+	struct kvm_steal_time *src;
+
+	src = &per_cpu(steal_time, cpu);
+
+	return !!src->preempted;
+}
+
 #ifdef CONFIG_SMP
 static void __init kvm_smp_prepare_boot_cpu(void)
 {
@@ -471,6 +480,9 @@ void __init kvm_guest_init(void)
 	if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
 		has_steal_clock = 1;
 		pv_time_ops.steal_clock = kvm_steal_clock;
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+		pv_lock_ops.vcpu_is_preempted = kvm_vcpu_is_preempted;
+#endif
 	}
 
 	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 10/11] x86, xen: support vcpu preempted check

From: Juergen Gross <jgross at suse.com>

Support the vcpu_is_preempted() functionality under Xen. This will
enhance lock performance on overcommitted hosts (more runnable vcpus
than physical cpus in the system) as doing busy waits for preempted
vcpus will hurt system performance far worse than early yielding.

A quick test (4 vcpus on 1 physical cpu doing a parallel build job
with "make -j 8") reduced system time by about 5% with this patch.

Signed-off-by: Juergen Gross <jgross at suse.com>
Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
---
 arch/x86/xen/spinlock.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 3d6e006..74756bb 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -114,7 +114,6 @@ void xen_uninit_lock_cpu(int cpu)
 	per_cpu(irq_name, cpu) = NULL;
 }
 
-
 /*
  * Our init of PV spinlocks is split in two init functions due to us
  * using paravirt patching and jump labels patching and having to do
@@ -137,6 +136,8 @@ void __init xen_init_spinlocks(void)
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
 	pv_lock_ops.wait = xen_qlock_wait;
 	pv_lock_ops.kick = xen_qlock_kick;
+
+	pv_lock_ops.vcpu_is_preempted = xen_vcpu_stolen;
 }
 
 /*
-- 
2.4.11

Pan Xinhui

2016-Nov-02 09:08 UTC

head link

[PATCH v7 11/11] Documentation: virtual: kvm: Support vcpu preempted check

Commit ("x86, kvm: support vcpu preempted check") add one field
"__u8
preempted" into struct kvm_steal_time. This field tells if one vcpu is
running or not.

It is zero if 1) some old KVM deos not support this filed. 2) the vcpu
is not preempted. Other values means the vcpu has been preempted.

Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
Acked-by: Radim Kr?m?? <rkrcmar at redhat.com>
Acked-by: Paolo Bonzini <pbonzini at redhat.com>
---
 Documentation/virtual/kvm/msr.txt | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/kvm/msr.txt
b/Documentation/virtual/kvm/msr.txt
index 2a71c8f..ab2ab76 100644
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -208,7 +208,9 @@ MSR_KVM_STEAL_TIME: 0x4b564d03
 		__u64 steal;
 		__u32 version;
 		__u32 flags;
-		__u32 pad[12];
+		__u8  preempted;
+		__u8  u8_pad[3];
+		__u32 pad[11];
 	}
 
 	whose data will be filled in by the hypervisor periodically. Only one
@@ -232,6 +234,11 @@ MSR_KVM_STEAL_TIME: 0x4b564d03
 		nanoseconds. Time during which the vcpu is idle, will not be
 		reported as steal time.
 
+		preempted: indicate the VCPU who owns this struct is running or
+		not. Non-zero values mean the VCPU has been preempted. Zero
+		means the VCPU is not preempted. NOTE, it is always zero if the
+		the hypervisor doesn't support this field.
+
 MSR_KVM_EOI_EN: 0x4b564d04
 	data: Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0
 	when disabled.  Bit 1 is reserved and must be zero.  When PV end of
-- 
2.4.11

Peter Zijlstra

2016-Nov-15 15:47 UTC

head link

[PATCH v7 06/11] x86, paravirt: Add interface to support kvm/xen vcpu preempted check

On Wed, Nov 02, 2016 at 05:08:33AM -0400, Pan Xinhui
wrote:> diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
> index 0f400c0..38c3bb7 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -310,6 +310,8 @@ struct pv_lock_ops {
>  
>  	void (*wait)(u8 *ptr, u8 val);
>  	void (*kick)(int cpu);
> +
> +	bool (*vcpu_is_preempted)(int cpu);
>  };
So that ends up with a full function call in the native case. I did
something like the below on top, completely untested, not been near a
compiler etc..

It doesn't get rid of the branch, but at least it avoids the function
call, and hardware should have no trouble predicting a constant
condition.

Also, it looks like you end up not setting vcpu_is_preempted when KVM
doesn't support steal clock, which would end up in an instant NULL
deref. Fixed that too.

---
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -673,6 +673,11 @@ static __always_inline void pv_kick(int
 	PVOP_VCALL1(pv_lock_ops.kick, cpu);
 }
 
+static __always_inline void pv_vcpu_is_prempted(int cpu)
+{
+	PVOP_VCALLEE1(pv_lock_ops.vcpu_is_preempted, cpu);
+}
+
 #endif /* SMP && PARAVIRT_SPINLOCKS */
 
 #ifdef CONFIG_X86_32
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -309,7 +309,7 @@ struct pv_lock_ops {
 	void (*wait)(u8 *ptr, u8 val);
 	void (*kick)(int cpu);
 
-	bool (*vcpu_is_preempted)(int cpu);
+	struct paravirt_callee_save vcpu_is_preempted;
 };
 
 /* This contains all the paravirt structures: we get a convenient
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -32,6 +32,12 @@ static inline void queued_spin_unlock(st
 {
 	pv_queued_spin_unlock(lock);
 }
+
+#define vcpu_is_preempted vcpu_is_preempted
+static inline bool vcpu_is_preempted(int cpu)
+{
+	return pv_vcpu_is_preempted(cpu);
+}
 #else
 static inline void queued_spin_unlock(struct qspinlock *lock)
 {
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -26,14 +26,6 @@
 extern struct static_key paravirt_ticketlocks_enabled;
 static __always_inline bool static_key_false(struct static_key *key);
 
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
-#define vcpu_is_preempted vcpu_is_preempted
-static inline bool vcpu_is_preempted(int cpu)
-{
-	return pv_lock_ops.vcpu_is_preempted(cpu);
-}
-#endif
-
 #include <asm/qspinlock.h>
 
 /*
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -415,15 +415,6 @@ void kvm_disable_steal_time(void)
 	wrmsr(MSR_KVM_STEAL_TIME, 0, 0);
 }
 
-static bool kvm_vcpu_is_preempted(int cpu)
-{
-	struct kvm_steal_time *src;
-
-	src = &per_cpu(steal_time, cpu);
-
-	return !!src->preempted;
-}
-
 #ifdef CONFIG_SMP
 static void __init kvm_smp_prepare_boot_cpu(void)
 {
@@ -480,9 +471,6 @@ void __init kvm_guest_init(void)
 	if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
 		has_steal_clock = 1;
 		pv_time_ops.steal_clock = kvm_steal_clock;
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
-		pv_lock_ops.vcpu_is_preempted = kvm_vcpu_is_preempted;
-#endif
 	}
 
 	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
@@ -604,6 +592,14 @@ static void kvm_wait(u8 *ptr, u8 val)
 	local_irq_restore(flags);
 }
 
+static bool __kvm_vcpu_is_preempted(int cpu)
+{
+	struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
+
+	return !!src->preempted;
+}
+PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
+
 /*
  * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
  */
@@ -620,6 +616,12 @@ void __init kvm_spinlock_init(void)
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
 	pv_lock_ops.wait = kvm_wait;
 	pv_lock_ops.kick = kvm_kick_cpu;
+	pv_lock_ops.vcpu_is_preempted = PV_CALLEE_SAVE(__native_vcpu_is_preempted);
+
+	if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
+		pv_lock_ops.vcpu_is_preempted +			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
+	}
 }
 
 static __init int kvm_spinlock_init_jump(void)
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -12,7 +12,6 @@ __visible void __native_queued_spin_unlo
 {
 	native_queued_spin_unlock(lock);
 }
-
 PV_CALLEE_SAVE_REGS_THUNK(__native_queued_spin_unlock);
 
 bool pv_is_native_spin_unlock(void)
@@ -21,9 +20,16 @@ bool pv_is_native_spin_unlock(void)
 		__raw_callee_save___native_queued_spin_unlock;
 }
 
-static bool native_vcpu_is_preempted(int cpu)
+__visible bool __native_vcpu_is_preempted(int cpu)
 {
-	return 0;
+	return false;
+}
+PV_CALLEE_SAVE_REGS_THUNK(__native_vcpu_is_preempted);
+
+bool pv_is_native_vcpu_is_preempted(void)
+{
+	return pv_lock_ops.queued_spin_unlock.func =+	
__raw_callee_save__native_vcpu_is_preempted;
 }
 
 struct pv_lock_ops pv_lock_ops = {
@@ -32,7 +38,7 @@ struct pv_lock_ops pv_lock_ops = {
 	.queued_spin_unlock = PV_CALLEE_SAVE(__native_queued_spin_unlock),
 	.wait = paravirt_nop,
 	.kick = paravirt_nop,
-	.vcpu_is_preempted = native_vcpu_is_preempted,
+	.vcpu_is_preempted = PV_CALLEE_SAVE(__native_vcpu_is_preempted),
 #endif /* SMP */
 };
 EXPORT_SYMBOL(pv_lock_ops);
--- a/arch/x86/kernel/paravirt_patch_32.c
+++ b/arch/x86/kernel/paravirt_patch_32.c
@@ -11,6 +11,7 @@ DEF_NATIVE(pv_mmu_ops, read_cr3, "mov %c
 
 #if defined(CONFIG_PARAVIRT_SPINLOCKS)
 DEF_NATIVE(pv_lock_ops, queued_spin_unlock, "movb $0, (%eax)");
+DEF_NATIVE(pv_lock_ops, vcpu_is_preempted, "movl $0, %eax");
 #endif
 
 unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
@@ -26,6 +27,7 @@ unsigned paravirt_patch_ident_64(void *i
 }
 
 extern bool pv_is_native_spin_unlock(void);
+extern bool pv_is_native_vcpu_is_preempted(void);
 
 unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
 		      unsigned long addr, unsigned len)
@@ -54,6 +56,12 @@ unsigned native_patch(u8 type, u16 clobb
 				end   = end_pv_lock_ops_queued_spin_unlock;
 				goto patch_site;
 			}
+		case PARAVIRT_PATCH(pv_lock_ops.vcpu_is_preempted):
+			if (pv_is_native_vcpu_is_preempted()) {
+				start = start_pv_lock_ops_vcpu_is_preempted;
+				end   = end_pv_lock_ops_vcpu_is_preempted;
+				goto patch_site;
+			}
 #endif
 
 	default:
--- a/arch/x86/kernel/paravirt_patch_64.c
+++ b/arch/x86/kernel/paravirt_patch_64.c
@@ -20,6 +20,7 @@ DEF_NATIVE(, mov64, "mov %rdi, %rax");
 
 #if defined(CONFIG_PARAVIRT_SPINLOCKS)
 DEF_NATIVE(pv_lock_ops, queued_spin_unlock, "movb $0, (%rdi)");
+DEF_NATIVE(pv_lock_ops, vcpu_is_preempted, "movl $0, rax");
 #endif
 
 unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
@@ -35,6 +36,7 @@ unsigned paravirt_patch_ident_64(void *i
 }
 
 extern bool pv_is_native_spin_unlock(void);
+extern bool pv_is_native_vcpu_is_preempted(void);
 
 unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
 		      unsigned long addr, unsigned len)
@@ -66,6 +68,12 @@ unsigned native_patch(u8 type, u16 clobb
 				end   = end_pv_lock_ops_queued_spin_unlock;
 				goto patch_site;
 			}
+		case PARAVIRT_PATCH(pv_lock_ops.vcpu_is_preempted):
+			if (pv_is_native_vcpu_is_preempted()) {
+				start = start_pv_lock_ops_vcpu_is_preempted;
+				end   = end_pv_lock_ops_vcpu_is_preempted;
+				goto patch_site;
+			}
 #endif
 
 	default:
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -114,6 +114,8 @@ void xen_uninit_lock_cpu(int cpu)
 	per_cpu(irq_name, cpu) = NULL;
 }
 
+PV_CALLEE_SAVE_REGS_THUNK(xen_vcpu_stolen);
+
 /*
  * Our init of PV spinlocks is split in two init functions due to us
  * using paravirt patching and jump labels patching and having to do
@@ -136,8 +138,7 @@ void __init xen_init_spinlocks(void)
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
 	pv_lock_ops.wait = xen_qlock_wait;
 	pv_lock_ops.kick = xen_qlock_kick;
-
-	pv_lock_ops.vcpu_is_preempted = xen_vcpu_stolen;
+	pv_lock_ops.vcpu_is_preempted = PV_CALLEE_SAVE(xen_vcpu_stolen);
 }
 
 /*

Andrea Arcangeli

2016-Dec-19 11:42 UTC

head link

[PATCH v7 08/11] x86, kvm/x86.c: support vcpu preempted check

Hello,

On Wed, Nov 02, 2016 at 05:08:35AM -0400, Pan Xinhui
wrote:> Support the vcpu_is_preempted() functionality under KVM. This will
> enhance lock performance on overcommitted hosts (more runnable vcpus
> than physical cpus in the system) as doing busy waits for preempted
> vcpus will hurt system performance far worse than early yielding.
> 
> Use one field of struct kvm_steal_time ::preempted to indicate that if
> one vcpu is running or not.
> 
> Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
> Acked-by: Paolo Bonzini <pbonzini at redhat.com>
> ---
>  arch/x86/include/uapi/asm/kvm_para.h |  4 +++-
>  arch/x86/kvm/x86.c                   | 16 ++++++++++++++++
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
[..]> +static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
> +{
> +	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
> +		return;
> +
> +	vcpu->arch.st.steal.preempted = 1;
> +
> +	kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime,
> +			&vcpu->arch.st.steal.preempted,
> +			offsetof(struct kvm_steal_time, preempted),
> +			sizeof(vcpu->arch.st.steal.preempted));
> +}
> +
>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
> +	kvm_steal_time_set_preempted(vcpu);
>  	kvm_x86_ops->vcpu_put(vcpu);
>  	kvm_put_guest_fpu(vcpu);
>  	vcpu->arch.last_host_tsc = rdtsc();
You can't call kvm_steal_time_set_preempted in atomic context (neither
in sched_out notifier nor in vcpu_put() after
preempt_disable)). __copy_to_user in kvm_write_guest_offset_cached
schedules and locks up the host.

kvm->srcu (or kvm->slots_lock) is also not taken and
kvm_write_guest_offset_cached needs to call kvm_memslots which
requires it.

This I think is why postcopy live migration locks up with current
upstream, and it doesn't seem related to userfaultfd at all (initially
I suspected the vmf conversion but it wasn't that) and in theory it
can happen with heavy swapping or page migration too.

Just the page is written so frequently it's unlikely to be swapped
out. The page being written so frequently also means it's very likely
found as re-dirtied when postcopy starts and that pretty much
guarantees an userfault will trigger a scheduling event in
kvm_steal_time_set_preempted in destination. There are opposite
probabilities of reproducing this with swapping vs postcopy live
migration.

For now I applied the below two patches, but this just will skip the
write and only prevent the host instability as nobody checks the
retval of __copy_to_user (what happens to guest after the write is
skipped is not as clear and should be investigated, but at least the
host will survive and not all guests will care about this flag being
updated). For this to be fully safe the preempted information should
be just an hint and not fundamental for correct functionality of the
guest pv spinlock code.

This bug was introduced in commit
0b9f6c4615c993d2b552e0d2bd1ade49b56e5beb in v4.9-rc7.
>From 458897fd44aa9b91459a006caa4051a7d1628a23 Mon Sep 17 00:00:00 2001From: Andrea Arcangeli <aarcange at redhat.com>
Date: Sat, 17 Dec 2016 18:43:52 +0100
Subject: [PATCH 1/2] kvm: fix schedule in atomic in
 kvm_steal_time_set_preempted()

kvm_steal_time_set_preempted() isn't disabling the pagefaults before
calling __copy_to_user and the kernel debug notices.

Signed-off-by: Andrea Arcangeli <aarcange at redhat.com>
---
 arch/x86/kvm/x86.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1f0d238..2dabaeb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2844,7 +2844,17 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu
*vcpu)
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
+	/*
+	 * Disable page faults because we're in atomic context here.
+	 * kvm_write_guest_offset_cached() would call might_fault()
+	 * that relies on pagefault_disable() to tell if there's a
+	 * bug. NOTE: the write to guest memory may not go through if
+	 * during postcopy live migration or if there's heavy guest
+	 * paging.
+	 */
+	pagefault_disable();
 	kvm_steal_time_set_preempted(vcpu);
+	pagefault_enable();
 	kvm_x86_ops->vcpu_put(vcpu);
 	kvm_put_guest_fpu(vcpu);
 	vcpu->arch.last_host_tsc = rdtsc();

>From 2845eba22ac74c5e313e3b590f9dac33e1b3cfef Mon Sep 17 00:00:00 2001From: Andrea Arcangeli <aarcange at redhat.com>
Date: Sat, 17 Dec 2016 19:13:32 +0100
Subject: [PATCH 2/2] kvm: take srcu lock around kvm_steal_time_set_preempted()

kvm_memslots() will be called by kvm_write_guest_offset_cached() so
take the srcu lock.

Signed-off-by: Andrea Arcangeli <aarcange at redhat.com>
---
 arch/x86/kvm/x86.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2dabaeb..02e6ab4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2844,6 +2844,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu
*vcpu)
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
+	int idx;
 	/*
 	 * Disable page faults because we're in atomic context here.
 	 * kvm_write_guest_offset_cached() would call might_fault()
@@ -2853,7 +2854,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	 * paging.
 	 */
 	pagefault_disable();
+	/*
+	 * kvm_memslots() will be called by
+	 * kvm_write_guest_offset_cached() so take the srcu lock.
+	 */
+	idx = srcu_read_lock(&vcpu->kvm->srcu);
 	kvm_steal_time_set_preempted(vcpu);
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 	pagefault_enable();
 	kvm_x86_ops->vcpu_put(vcpu);
 	kvm_put_guest_fpu(vcpu);

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