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[PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

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  [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state Waiman Long <Waiman.Long@hpe.com> - 2016-02-12 18:40 +0100
    Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer  in a sleeping state Peter Zijlstra <peterz@infradead.org> - 2016-02-12 21:20 +0100
      Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer  in a sleeping state Waiman Long <waiman.long@hpe.com> - 2016-02-12 22:40 +0100
        Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer  in a sleeping state Peter Zijlstra <peterz@infradead.org> - 2016-02-13 13:20 +0100

#1332959 — [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

When a task prepares to sleep and then aborts it somehow, there is
a small chance that a waker may be spinning on the on_cpu flag of
that task waiting for the flag to turn off before doing the wakeup
operation. It may keep on spinning for a long time until that task
actually sleeps leading to spurious wakeup.

This patch adds code to detect the change in task state and abort
the wakeup operation, when appropriate, to free up the waker's cpu
to do other useful works.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
 kernel/sched/core.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7e548bd..e4b6e84 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2075,8 +2075,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	 *
 	 * This ensures that tasks getting woken will be fully ordered against
 	 * their previous state and preserve Program Order.
+	 *
+	 * If the owning cpu decides not to sleep after all by changing back
+	 * its task state, we can return immediately.
 	 */
-	smp_cond_acquire(!p->on_cpu);
+	smp_cond_acquire(!p->on_cpu || !(p->state & state));
+	if (!(p->state & state)) {
+		success = 0;
+		goto out;
+	}
 
 	p->sched_contributes_to_load = !!task_contributes_to_load(p);
 	p->state = TASK_WAKING;
-- 
1.7.1

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#1333082 — Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

On Fri, Feb 12, 2016 at 12:32:15PM -0500, Waiman Long wrote:
> When a task prepares to sleep and then aborts it somehow, there is
> a small chance that a waker may be spinning on the on_cpu flag of
> that task waiting for the flag to turn off before doing the wakeup
> operation. It may keep on spinning for a long time until that task
> actually sleeps leading to spurious wakeup.
> 
> This patch adds code to detect the change in task state and abort
> the wakeup operation, when appropriate, to free up the waker's cpu
> to do other useful works.
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> ---
>  kernel/sched/core.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7e548bd..e4b6e84 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2075,8 +2075,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  	 *
>  	 * This ensures that tasks getting woken will be fully ordered against
>  	 * their previous state and preserve Program Order.
> +	 *
> +	 * If the owning cpu decides not to sleep after all by changing back
> +	 * its task state, we can return immediately.
>  	 */
> -	smp_cond_acquire(!p->on_cpu);
> +	smp_cond_acquire(!p->on_cpu || !(p->state & state));
> +	if (!(p->state & state)) {
> +		success = 0;
> +		goto out;
> +	}

This doesn't make sense, if we managed to get here, p->on_rq must be
false, which means the other side is already in the middle of
schedule().

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#1333145 — Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

On 02/12/2016 03:18 PM, Peter Zijlstra wrote:
> On Fri, Feb 12, 2016 at 12:32:15PM -0500, Waiman Long wrote:
>> When a task prepares to sleep and then aborts it somehow, there is
>> a small chance that a waker may be spinning on the on_cpu flag of
>> that task waiting for the flag to turn off before doing the wakeup
>> operation. It may keep on spinning for a long time until that task
>> actually sleeps leading to spurious wakeup.
>>
>> This patch adds code to detect the change in task state and abort
>> the wakeup operation, when appropriate, to free up the waker's cpu
>> to do other useful works.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@hp.com>
>> ---
>>   kernel/sched/core.c |    9 ++++++++-
>>   1 files changed, 8 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 7e548bd..e4b6e84 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2075,8 +2075,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>>   	 *
>>   	 * This ensures that tasks getting woken will be fully ordered against
>>   	 * their previous state and preserve Program Order.
>> +	 *
>> +	 * If the owning cpu decides not to sleep after all by changing back
>> +	 * its task state, we can return immediately.
>>   	 */
>> -	smp_cond_acquire(!p->on_cpu);
>> +	smp_cond_acquire(!p->on_cpu || !(p->state&  state));
>> +	if (!(p->state&  state)) {
>> +		success = 0;
>> +		goto out;
>> +	}
> This doesn't make sense, if we managed to get here, p->on_rq must be
> false, which means the other side is already in the middle of
> schedule().
Yes, you are right. It is my bad that I miss the on_rq check earlier. 
Just scrap the last patch.

Sorry for that:-[
Longman

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#1333311 — Re: [PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

On Fri, Feb 12, 2016 at 04:22:29PM -0500, Waiman Long wrote:

> >>+	smp_cond_acquire(!p->on_cpu || !(p->state&  state));
> >>+	if (!(p->state&  state)) {
> >>+		success = 0;
> >>+		goto out;
> >>+	}
> >This doesn't make sense, if we managed to get here, p->on_rq must be
> >false, which means the other side is already in the middle of
> >schedule().
> Yes, you are right. It is my bad that I miss the on_rq check earlier. Just
> scrap the last patch.

No worries, that is tricky code :-)

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