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

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Date:	Fri, 12 Feb 2016 21:18:45 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: 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@...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().