linux-kernel - Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle

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Date:   Wed, 13 Sep 2023 11:52:14 +0530
From:   "Gautham R. Shenoy" <gautham.shenoy@....com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Aaron Lu <aaron.lu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in
 select_idle_cpu()

On Mon, Sep 11, 2023 at 04:40:02PM +0800, Chen Yu wrote:
> Hi Aaron,
> 
> thanks for the review,
> 
> On 2023-09-11 at 15:26:29 +0800, Aaron Lu wrote:

[..snip..]

> > > @@ -6982,8 +6997,13 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
> > >  static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> > >  {
> > >  	if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
> > > -	    sched_cpu_cookie_match(cpu_rq(cpu), p))
> > > +	    sched_cpu_cookie_match(cpu_rq(cpu), p)) {
> > > +		if (sched_feat(SIS_CACHE) &&
> > > +		    sched_clock_cpu(cpu) < cpu_rq(cpu)->cache_hot_timeout)
> > > +			return -1;
> > > +
> > 
> > Maybe introduce a new function that also considers rq->cache_hot_timeout,
> > like available_idle_cpu_migrate() so that above and below logic can be
> > simplified a bit?
> > 
> 
> Yes, that would be simpler, I'll do in next version.
> 
> > I was thinking to simply add that rq->cache_hot_timeout check to
> > available_idle_cpu() but then a long sleeping task could be forced to
> > migrate if its prev_cpu happens to just deschedule a task that sets rq's
> > cache_hot_timeout. I guess that's why you chose to only change the idle
> > semantic in select_idle_cpu() but not in select_idle_sibling()?
> >
> 
> Yes, sort of. And the reason I did not put this cache hot check in available_idle_cpu()
> or idle_cpu() was mainly because these APIs are generic and could be invoked by select_idle_sibling().
> If the task fall asleep and woken up quickly, its previous idle CPU will also be skipped,
> thus no one could use this CPU within the cache hot period, including the cache-hot task
> itself.

This happens even with this patch right? It is possible for a task p1
whose avg sleep time is "t" to go to sleep which causes its CPU to go
idle. When it wakes up after a time t' < t, the logic above skips the
idle CPU because it is still cache-hot, despite the fact that it is
cache hot for p1!

Have you considered recording p1's identity in the
rq->cache_hot_sleeper so that in select_task_rq_fair(), we can simply
return the previous CPU if it is cache hot and the wakee is
rq->cache_hot_sleeper, thus avoiding the whole select_idle_sibling
scan.

> 
> thanks,
> Chenyu

--
Thanks and Regards
gautham.