Re: [PATCH] sched: access local runqueue directly in single_task_running

[Dominik Dingel]

On Fri, 18 Sep 2015 13:26:53 +0200
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> 
> 
> On 18/09/2015 11:27, Dominik Dingel wrote:
> > Commit 2ee507c47293 ("sched: Add function single_task_running to let a task
> > check if it is the only task running on a cpu") referenced the current
> > runqueue with the smp_processor_id.  When CONFIG_DEBUG_PREEMPT is enabled,
> > that is only allowed if preemption is disabled or the currrent task is
> > bound to the local cpu (e.g. kernel worker).
> > 
> > With commit f78195129963 ("kvm: add halt_poll_ns module parameter") KVM
> > calls single_task_running. If CONFIG_DEBUG_PREEMPT is enabled that
> > generates a lot of kernel messages.
> > 
> > To avoid adding preemption in that cases, as it would limit the usefulness,
> > we change single_task_running to access directly the cpu local runqueue.
> > 
> > Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 4.2.x
> > Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/core.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 78b4bad10..5bfad0b 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2614,13 +2614,13 @@ unsigned long nr_running(void)
> >  
> >  /*
> >   * Check if only the current task is running on the cpu.
> > + *
> > + * Caution result is subject to time-of-check-to-time-of-use race,
> > + * every caller is responsible to set up additional fences if necessary.
> 
> Let's expand it a bit more:
> 
>  * Caution: this function does not check that the caller has disabled
>  * preemption, thus the result might have a time-of-check-to-time-of-use
>  * race.  The caller is responsible to use this correctly, for example:
>  *
>  * - use it from a non-preemptable section
>  *
>  * - use it from a thread that is bound to a single CPU
>  *
>  * - use it in a loop where each iteration takes very little time
>  *   (e.g. a polling loop)
>  */
> 
> I'll include it in my pull request.

Sounds really good.
Thank you!

> Paolo
> 
> >   */
> >  bool single_task_running(void)
> >  {
> > -	if (cpu_rq(smp_processor_id())->nr_running == 1)
> > -		return true;
> > -	else
> > -		return false;
> > +	return raw_rq()->nr_running == 1;
> >  }
> >  EXPORT_SYMBOL(single_task_running);
> >  
> > 
> 

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