Re: [PATCH v2 1/8] sched/fair: Update affine statistics when needed

From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Rik van Riel <riel@surriel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Gautham R Shenoy <ego@linux.vnet.ibm.com>,
	Parth Shah <parth@linux.ibm.com>
Subject: Re: [PATCH v2 1/8] sched/fair: Update affine statistics when needed
Date: Tue, 11 May 2021 21:52:47 +0530	[thread overview]
Message-ID: <20210511162247.GS2633526@linux.vnet.ibm.com> (raw)
In-Reply-To: <87sg2t1o9z.mognet@arm.com>

* Valentin Schneider <valentin.schneider@arm.com> [2021-05-11 12:51:52]:

> On 07/05/21 22:35, Srikar Dronamraju wrote:
> > * Valentin Schneider <valentin.schneider@arm.com> [2021-05-07 17:08:17]:
> >
> >> On 06/05/21 22:15, Srikar Dronamraju wrote:
> >> > wake_affine_idle() can return prev_cpu. Even in such a scenario,
> >> > scheduler was going ahead and updating schedstats related to wake
> >> > affine. i.e even if the task is not moved across LLC domains,
> >> > schedstats would have accounted.
> >
<snip>
> > Lets say if prev CPU and this CPU were part of the same LLC, and the prev
> > CPU was busy (or busier than this CPU), should consider this as a wake
> > affine? If prev was idle, we would have surely consider prev CPU. Also since
> > both are part of same LLC, we cant say this CPU is more affine than prev
> > CPU. Or may be I am confusing wake_affine with cache_affine.
> >
> 
> SD_WAKE_AFFINE says: "Consider waking task on waking CPU.", with that I
> read wake_affine() as: "should I place the wakee close to the waker or
> close to its previous CPU?". This can be yes or no even if both are in the
> same LLC.
> 

Okay.

<snip>

> >> > --- a/kernel/sched/fair.c
> >> > +++ b/kernel/sched/fair.c
> >> > @@ -5884,8 +5884,10 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> >> >       if (target == nr_cpumask_bits)
> >> >               return prev_cpu;
> >> >
> >> > -	schedstat_inc(sd->ttwu_move_affine);
> >> > -	schedstat_inc(p->se.statistics.nr_wakeups_affine);
> >> > +	if (!cpus_share_cache(prev_cpu, target)) {
> >>
> >> Per the above, why? Why not just if(target == this_cpu) ?
> >
> > We could use target == this_cpu. However if prev CPU and this CPU share the
> > same LLC, then should we consider moving to this_cpu as an affine wakeup?
> >
> 
> It would make sense if it's a sync wakeup, which wake_affine() does try to
> do ATM (regardless of LLC actually, if I'm reading it correctly).

Okay, I will replace the cpus_share_cache check with target == this_cpu.

-- 
Thanks and Regards
Srikar Dronamraju