[PATCH v2] locking/osq: Use optimized spinning loop for arm64

[PATCH v2] locking/osq: Use optimized spinning loop for arm64

[Waiman Long]

Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
for spinlock that can boost performance of sibling threads by putting
the current cpu to a shallow sleep state that is woken up only when
the monitored variable changes or an external event happens.

OSQ has a more complicated spinning loop. Besides the lock value, it
also checks for need_resched() and vcpu_is_preempted(). The check for
need_resched() is not a problem as it is only set by the tick interrupt
handler. That will be detected by the spinning cpu right after iret.

The vcpu_is_preempted() check, however, is a problem as changes to the
preempt state of of previous node will not affect the sleep state. For
ARM64, vcpu_is_preempted is not defined and so is a no-op. To guard
against future addition of vcpu_is_preempted() to arm64, code is added
to cause build error when vcpu_is_preempted becomes defined in arm64
without the corresponding changes in the OSQ spinning code.

On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking
microbenchmark was run for 10s with and without the patch. The
performance numbers before patch were:

Running locktest with mutex [runtime = 10s, load = 1]
Threads = 224, Min/Mean/Max = 316/123,143/2,121,269
Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s

After patch, the numbers were:

Running locktest with mutex [runtime = 10s, load = 1]
Threads = 224, Min/Mean/Max = 334/147,836/1,304,787
Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s

So there was about 20% performance improvement.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/arm64/include/asm/barrier.h | 10 ++++++++++
 kernel/locking/osq_lock.c        | 25 ++++++++++++-------------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 7d9cc5ec4971..8eb5f1239885 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -152,6 +152,16 @@ do {									\
 	VAL;								\
 })
 
+/*
+ * In osq_lock(), smp_cond_load_relaxed() is called with a condition
+ * that includes vcpu_is_preempted(). For arm64, vcpu_is_preempted is not
+ * currently defined. So it is a no-op. If vcpu_is_preempted is defined in
+ * the future, smp_cond_load_relaxed() will not response to changes in the
+ * preempt state in a timely manner. So code changes will have to be made
+ * to address this deficiency.
+ */
+#define vcpu_is_preempted_not_used
+
 #define smp_cond_load_acquire(ptr, cond_expr)				\
 ({									\
 	typeof(ptr) __PTR = (ptr);					\
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..69ec5161c3cc 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -13,6 +13,14 @@
  */
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
 
+/*
+ * The optimized smp_cond_load_relaxed() spin loop should not be used with
+ * vcpu_is_preempted defined.
+ */
+#if defined(vcpu_is_preempted) && defined(vcpu_is_preempted_not_used)
+#error "vcpu_is_preempted() inside smp_cond_load_relaxed() may not work!"
+#endif
+
 /*
  * We use the value 0 to represent "no CPU", thus the encoded value
  * will be the CPU number incremented by 1.
@@ -134,20 +142,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * cmpxchg in an attempt to undo our queueing.
 	 */
 
-	while (!READ_ONCE(node->locked)) {
-		/*
-		 * If we need to reschedule bail... so we can block.
-		 * Use vcpu_is_preempted() to avoid waiting for a preempted
-		 * lock holder:
-		 */
-		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
-			goto unqueue;
-
-		cpu_relax();
-	}
-	return true;
+	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
+				  vcpu_is_preempted(node_cpu(node->prev))))
+		return true;
 
-unqueue:
+	/* unqueue */
 	/*
 	 * Step - A  -- stabilize @prev
 	 *
-- 
2.18.1

Re: [PATCH v2] locking/osq: Use optimized spinning loop for arm64

[yezengruan]

Hi Waiman,

On 2020/1/13 7:58, Waiman Long wrote:
> Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
> for spinlock that can boost performance of sibling threads by putting
> the current cpu to a shallow sleep state that is woken up only when
> the monitored variable changes or an external event happens.
> 
> OSQ has a more complicated spinning loop. Besides the lock value, it
> also checks for need_resched() and vcpu_is_preempted(). The check for
> need_resched() is not a problem as it is only set by the tick interrupt
> handler. That will be detected by the spinning cpu right after iret.
> 
> The vcpu_is_preempted() check, however, is a problem as changes to the
> preempt state of of previous node will not affect the sleep state. For
> ARM64, vcpu_is_preempted is not defined and so is a no-op. To guard
> against future addition of vcpu_is_preempted() to arm64, code is added
> to cause build error when vcpu_is_preempted becomes defined in arm64
> without the corresponding changes in the OSQ spinning code.

Recently, I am supporting vcpu_is_preempted() for arm64. There is a patch set which do this[1].

[1] https://lore.kernel.org/linux-arm-kernel/20191226135833.1052-1-yezengruan@huawei.com/

> 
> On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking
> microbenchmark was run for 10s with and without the patch. The
> performance numbers before patch were:
> 
> Running locktest with mutex [runtime = 10s, load = 1]
> Threads = 224, Min/Mean/Max = 316/123,143/2,121,269
> Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s
> 
> After patch, the numbers were:
> 
> Running locktest with mutex [runtime = 10s, load = 1]
> Threads = 224, Min/Mean/Max = 334/147,836/1,304,787
> Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s
> 
> So there was about 20% performance improvement.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/arm64/include/asm/barrier.h | 10 ++++++++++
>  kernel/locking/osq_lock.c        | 25 ++++++++++++-------------
>  2 files changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 7d9cc5ec4971..8eb5f1239885 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -152,6 +152,16 @@ do {									\
>  	VAL;								\
>  })
>  
> +/*
> + * In osq_lock(), smp_cond_load_relaxed() is called with a condition
> + * that includes vcpu_is_preempted(). For arm64, vcpu_is_preempted is not
> + * currently defined. So it is a no-op. If vcpu_is_preempted is defined in
> + * the future, smp_cond_load_relaxed() will not response to changes in the
> + * preempt state in a timely manner. So code changes will have to be made
> + * to address this deficiency.
> + */
> +#define vcpu_is_preempted_not_used
> +
>  #define smp_cond_load_acquire(ptr, cond_expr)				\
>  ({									\
>  	typeof(ptr) __PTR = (ptr);					\
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 6ef600aa0f47..69ec5161c3cc 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -13,6 +13,14 @@
>   */
>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
>  
> +/*
> + * The optimized smp_cond_load_relaxed() spin loop should not be used with
> + * vcpu_is_preempted defined.
> + */
> +#if defined(vcpu_is_preempted) && defined(vcpu_is_preempted_not_used)
> +#error "vcpu_is_preempted() inside smp_cond_load_relaxed() may not work!"
> +#endif
> +
>  /*
>   * We use the value 0 to represent "no CPU", thus the encoded value
>   * will be the CPU number incremented by 1.
> @@ -134,20 +142,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * cmpxchg in an attempt to undo our queueing.
>  	 */
>  
> -	while (!READ_ONCE(node->locked)) {
> -		/*
> -		 * If we need to reschedule bail... so we can block.
> -		 * Use vcpu_is_preempted() to avoid waiting for a preempted
> -		 * lock holder:
> -		 */
> -		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
> -			goto unqueue;
> -
> -		cpu_relax();
> -	}
> -	return true;
> +	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> +				  vcpu_is_preempted(node_cpu(node->prev))))
> +		return true;
>  
> -unqueue:
> +	/* unqueue */
>  	/*
>  	 * Step - A  -- stabilize @prev
>  	 *
> 

Thanks,

Zengruan

Re: [PATCH v2] locking/osq: Use optimized spinning loop for arm64

[Will Deacon]

[+Marc]

On Sun, Jan 12, 2020 at 06:58:54PM -0500, Waiman Long wrote:
> Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
> for spinlock that can boost performance of sibling threads by putting
> the current cpu to a shallow sleep state that is woken up only when
> the monitored variable changes or an external event happens.
> 
> OSQ has a more complicated spinning loop. Besides the lock value, it
> also checks for need_resched() and vcpu_is_preempted(). The check for
> need_resched() is not a problem as it is only set by the tick interrupt
> handler. That will be detected by the spinning cpu right after iret.
> 
> The vcpu_is_preempted() check, however, is a problem as changes to the
> preempt state of of previous node will not affect the sleep state. For
> ARM64, vcpu_is_preempted is not defined and so is a no-op. To guard
> against future addition of vcpu_is_preempted() to arm64, code is added
> to cause build error when vcpu_is_preempted becomes defined in arm64
> without the corresponding changes in the OSQ spinning code.
> 
> On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking
> microbenchmark was run for 10s with and without the patch. The
> performance numbers before patch were:
> 
> Running locktest with mutex [runtime = 10s, load = 1]
> Threads = 224, Min/Mean/Max = 316/123,143/2,121,269
> Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s
> 
> After patch, the numbers were:
> 
> Running locktest with mutex [runtime = 10s, load = 1]
> Threads = 224, Min/Mean/Max = 334/147,836/1,304,787
> Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s
> 
> So there was about 20% performance improvement.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/arm64/include/asm/barrier.h | 10 ++++++++++
>  kernel/locking/osq_lock.c        | 25 ++++++++++++-------------
>  2 files changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 7d9cc5ec4971..8eb5f1239885 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -152,6 +152,16 @@ do {									\
>  	VAL;								\
>  })
>  
> +/*
> + * In osq_lock(), smp_cond_load_relaxed() is called with a condition
> + * that includes vcpu_is_preempted(). For arm64, vcpu_is_preempted is not
> + * currently defined. So it is a no-op. If vcpu_is_preempted is defined in
> + * the future, smp_cond_load_relaxed() will not response to changes in the
> + * preempt state in a timely manner. So code changes will have to be made
> + * to address this deficiency.
> + */
> +#define vcpu_is_preempted_not_used
> +
>  #define smp_cond_load_acquire(ptr, cond_expr)				\
>  ({									\
>  	typeof(ptr) __PTR = (ptr);					\
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 6ef600aa0f47..69ec5161c3cc 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -13,6 +13,14 @@
>   */
>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
>  
> +/*
> + * The optimized smp_cond_load_relaxed() spin loop should not be used with
> + * vcpu_is_preempted defined.
> + */
> +#if defined(vcpu_is_preempted) && defined(vcpu_is_preempted_not_used)
> +#error "vcpu_is_preempted() inside smp_cond_load_relaxed() may not work!"
> +#endif

Although I appreciate you going the extra mile for arm64 (thanks!), I think
this is probably a bit overkill given that I don't plan to merge the series
from Zengruan any time soon. Instead, how about just defining
vcpu_is_preempted in arch/arm64/include/asm/spinlock.h with a comment:


	/*
	 * Changing this will break osq_lock() thanks to the call inside
	 * smp_cond_load_relaxed().
	 *
	 * See:
	 * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
	 */
	#define vcpu_is_preempted(cpu)	false


So we'll notice that when somebody tries to change it.

Will

Re: [PATCH v2] locking/osq: Use optimized spinning loop for arm64

[Waiman Long]

On 1/13/20 6:57 AM, Will Deacon wrote:
> [+Marc]
>
> On Sun, Jan 12, 2020 at 06:58:54PM -0500, Waiman Long wrote:
>> Arm64 has a more optimized spinning loop (atomic_cond_read_acquire)
>> for spinlock that can boost performance of sibling threads by putting
>> the current cpu to a shallow sleep state that is woken up only when
>> the monitored variable changes or an external event happens.
>>
>> OSQ has a more complicated spinning loop. Besides the lock value, it
>> also checks for need_resched() and vcpu_is_preempted(). The check for
>> need_resched() is not a problem as it is only set by the tick interrupt
>> handler. That will be detected by the spinning cpu right after iret.
>>
>> The vcpu_is_preempted() check, however, is a problem as changes to the
>> preempt state of of previous node will not affect the sleep state. For
>> ARM64, vcpu_is_preempted is not defined and so is a no-op. To guard
>> against future addition of vcpu_is_preempted() to arm64, code is added
>> to cause build error when vcpu_is_preempted becomes defined in arm64
>> without the corresponding changes in the OSQ spinning code.
>>
>> On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking
>> microbenchmark was run for 10s with and without the patch. The
>> performance numbers before patch were:
>>
>> Running locktest with mutex [runtime = 10s, load = 1]
>> Threads = 224, Min/Mean/Max = 316/123,143/2,121,269
>> Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s
>>
>> After patch, the numbers were:
>>
>> Running locktest with mutex [runtime = 10s, load = 1]
>> Threads = 224, Min/Mean/Max = 334/147,836/1,304,787
>> Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s
>>
>> So there was about 20% performance improvement.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  arch/arm64/include/asm/barrier.h | 10 ++++++++++
>>  kernel/locking/osq_lock.c        | 25 ++++++++++++-------------
>>  2 files changed, 22 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index 7d9cc5ec4971..8eb5f1239885 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -152,6 +152,16 @@ do {									\
>>  	VAL;								\
>>  })
>>  
>> +/*
>> + * In osq_lock(), smp_cond_load_relaxed() is called with a condition
>> + * that includes vcpu_is_preempted(). For arm64, vcpu_is_preempted is not
>> + * currently defined. So it is a no-op. If vcpu_is_preempted is defined in
>> + * the future, smp_cond_load_relaxed() will not response to changes in the
>> + * preempt state in a timely manner. So code changes will have to be made
>> + * to address this deficiency.
>> + */
>> +#define vcpu_is_preempted_not_used
>> +
>>  #define smp_cond_load_acquire(ptr, cond_expr)				\
>>  ({									\
>>  	typeof(ptr) __PTR = (ptr);					\
>> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
>> index 6ef600aa0f47..69ec5161c3cc 100644
>> --- a/kernel/locking/osq_lock.c
>> +++ b/kernel/locking/osq_lock.c
>> @@ -13,6 +13,14 @@
>>   */
>>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
>>  
>> +/*
>> + * The optimized smp_cond_load_relaxed() spin loop should not be used with
>> + * vcpu_is_preempted defined.
>> + */
>> +#if defined(vcpu_is_preempted) && defined(vcpu_is_preempted_not_used)
>> +#error "vcpu_is_preempted() inside smp_cond_load_relaxed() may not work!"
>> +#endif
> Although I appreciate you going the extra mile for arm64 (thanks!), I think
> this is probably a bit overkill given that I don't plan to merge the series
> from Zengruan any time soon. Instead, how about just defining
> vcpu_is_preempted in arch/arm64/include/asm/spinlock.h with a comment:
>
>
> 	/*
> 	 * Changing this will break osq_lock() thanks to the call inside
> 	 * smp_cond_load_relaxed().
> 	 *
> 	 * See:
> 	 * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
> 	 */
> 	#define vcpu_is_preempted(cpu)	false
>
>
> So we'll notice that when somebody tries to change it.

Yes, that works for me. I just want to make sure that if any changes to
add vcpu_is_preempted to arm64 in the future will get caught.

Cheers,
Longman