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

[Waiman Long]

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@xxxxxx>
> > ---
> >   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