Re: [PATCH v2] sched: Queue task on wakelist in the same llc if the wakee cpu is idle

[Tianchen Ding]

On 2022/6/1 18:58, Valentin Schneider wrote:
> On 01/06/22 13:54, Tianchen Ding wrote:
> > On 2022/5/31 23:56, Valentin Schneider wrote:
> >
> > > Thanks!
> > >
> > > So I'm thinking we could first make that into
> > >
> > > 	if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
> > >
> > > Then building on this, we can generalize using the wakelist to any remote
> > > idle CPU (which on paper isn't as much as a clear win as just WF_ON_CPU,
> > > depending on how deeply idle the CPU is...)
> > >
> > > We need the cpu != this_cpu check, as that's currently served by the
> > > WF_ON_CPU check (AFAIU we can only observe p->on_cpu in there for remote
> > > tasks).
> > >
> > > ---
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 66c4e5922fe1..60038743f2f1 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -3830,13 +3830,20 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
> > >    	if (!cpus_share_cache(smp_processor_id(), cpu))
> > >    		return true;
> > >    
> > > +	if (cpu == smp_processor_id())
> > > +		return false;
> > > +
> > >    	/*
> > >    	 * If the task is descheduling and the only running task on the
> > >    	 * CPU then use the wakelist to offload the task activation to
> > >    	 * the soon-to-be-idle CPU as the current CPU is likely busy.
> > >    	 * nr_running is checked to avoid unnecessary task stacking.
> > > +	 *
> > > +	 * Note that we can only get here with (wakee) p->on_rq=0,
> > > +	 * p->on_cpu can be whatever, we've done the dequeue, so
> > > +	 * the wakee has been accounted out of ->nr_running
> > >    	 */
> > > -	if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <= 1)
> > > +	if (!cpu_rq(cpu)->nr_running)
> > >    		return true;
> > >    
> > >    	return false;
> >
> > Hi Valentin. I've done a simple unixbench test (Pipe-based Context
> > Switching) on my x86 machine with full threads (104).
> >
> >                old            patch1           patch1+patch2
> > score       7825.4     7500(more)-8000          9061.6
> >
> > patch1: use !cpu_rq(cpu)->nr_running instead of cpu_rq(cpu)->nr_running <= 1
> > patch2: ignore WF_ON_CPU check
> >
> > The score of patch1 is not stable. I've tested for many times and the
> > score is floating between about 7500-8000 (more at 7500).
> >
> > patch1 means more strict limit on using wakelist. But it may cause
> > performance regression.
> >
> > It seems that, using wakelist properly can help improve wakeup
> > performance, but using it too much may cause more IPIs. It's a trade-off
> > about how strict the ttwu_queue_cond() is.
> >
> > Anyhow, I think patch2 should be a pure improvement. What's your idea?
>
> Thanks for separately testing these two.
>
> I take it the results for patch1 are noticeably more swingy than the
> baseline? (FWIW boxplots are usually a nice way to summarize those sort of
> results).

Hmm... I'm not familiar with this...
T want to say that I'm not sure about the performance impact about 
patch1. While from the view of logic, patch1 should be correct.

> WF_ON_CPU && nr_running == 1 means the wakee is scheduling out *and* there
> is another task queued, I'm guessing that's relatively common in your
> unixbench scenario...
>
> Either way, I think we want to keep the two changes separate for the sake
> of testing and bisecting.

Yes. I'll split the patch to 2 parts. One for logic fix and another for 
performance improvement.