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

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Date:   Wed, 10 Nov 2021 17:04:14 +0800
From:   Zhaoyang Huang <huangzhaoyang@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Resend PATCH] psi : calc cfs task memstall time more precisely

On Wed, Nov 10, 2021 at 4:49 PM Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> 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
thanks vincent. Could rq_clock_task help on removing the preempted
time of CFS task by RT/DL, which is the mainly part we want to solve
on memstall time.
>
> >
> > Randomly scaling time as proposed seems almost certainly wrong. What
> > would that make the stats mean?