linux-kernel - Re: [Resend PATCH] psi : calc cfs task memstall time more precisely

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Date:   Wed, 10 Nov 2021 09:49:44 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Huangzhaoyang <huangzhaoyang@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Resend PATCH] psi : calc cfs task memstall time more precisely

On Tue, 9 Nov 2021 at 15:56, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Nov 02, 2021 at 03:47:33PM -0400, Johannes Weiner wrote:
> > CC peterz as well for rt and timekeeping magic
> >
> > On Fri, Oct 15, 2021 at 02:16:52PM +0800, Huangzhaoyang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > >
> > > In an EAS enabled system, there are two scenarios discordant to current design,
> > >
> > > 1. workload used to be heavy uneven among cores for sake of scheduler policy.
> > > RT task usually preempts CFS task in little core.
> > > 2. CFS task's memstall time is counted as simple as exit - entry so far, which
> > > ignore the preempted time by RT, DL and Irqs.
>
> It ignores preemption full-stop. I don't see why RT/IRQ should be
> special cased here.
>
> > > With these two constraints, the percpu nonidle time would be mainly consumed by
> > > none CFS tasks and couldn't be averaged. Eliminating them by calc the time growth
> > > via the proportion of cfs_rq's utilization on the whole rq.
>
>
> > > +static unsigned long psi_memtime_fixup(u32 growth)
> > > +{
> > > +   struct rq *rq = task_rq(current);
> > > +   unsigned long growth_fixed = (unsigned long)growth;
> > > +
> > > +   if (!(current->policy == SCHED_NORMAL || current->policy == SCHED_BATCH))
> > > +           return growth_fixed;
> > > +
> > > +   if (current->in_memstall)
> > > +           growth_fixed = div64_ul((1024 - rq->avg_rt.util_avg - rq->avg_dl.util_avg
> > > +                                   - rq->avg_irq.util_avg + 1) * growth, 1024);
> > > +
> > > +   return growth_fixed;
> > > +}
> > > +
> > >  static void init_triggers(struct psi_group *group, u64 now)
> > >  {
> > >     struct psi_trigger *t;
> > > @@ -658,6 +675,7 @@ static void record_times(struct psi_group_cpu *groupc, u64 now)
> > >     }
> > >
> > >     if (groupc->state_mask & (1 << PSI_MEM_SOME)) {
> > > +           delta = psi_memtime_fixup(delta);
> >
> > Ok, so we want to deduct IRQ and RT preemption time from the memstall
> > period of an active reclaimer, since it's technically not stalled on
> > memory during this time but on CPU.
> >
> > However, we do NOT want to deduct IRQ and RT time from memstalls that
> > are sleeping on refaults swapins, since they are not affected by what
> > is going on on the CPU.
>
> I think that focus on RT/IRQ is mis-guided here, and the implementation
> is horrendous.
>
> So the fundamental question seems to be; and I think Johannes is the one
> to answer that: What time-base do these metrics want to use?
>
> Do some of these states want to account in task-time instead of
> wall-time perhaps? I can't quite remember, but vague memories are
> telling me most of the PSI accounting was about blocked tasks, not
> running tasks, which makes all this rather more complicated.

I tend to agree with this.
Using rq_clock_task(rq) instead of cpu_clock(cpu) will remove the time
spent under interrupt as an example
and AFAICT, rq->clock_task is updated before calling psi function

>
> Randomly scaling time as proposed seems almost certainly wrong. What
> would that make the stats mean?