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

[Zhaoyang Huang]

On Wed, Nov 3, 2021 at 3:47 AM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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.
> >
> > 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.
> >
> > eg.
> > Here is the scenario which this commit want to fix, that is the rt and irq consume
> > some utilization of the whole rq. This scenario could be typical in a core
> > which is assigned to deal with all irqs. Furthermore, the rt task used to run on
> > little core under EAS.
> >
> > Binder:305_3-314    [002] d..1   257.880195: psi_memtime_fixup: original:30616,adjusted:25951,se:89,cfs:353,rt:139,dl:0,irq:18
> > droid.phone-1525    [001] d..1   265.145492: psi_memtime_fixup: original:61616,adjusted:53492,se:55,cfs:225,rt:121,dl:0,irq:15
> >
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > ---
> >  kernel/sched/psi.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index cc25a3c..754a836 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -182,6 +182,8 @@ struct psi_group psi_system = {
> >
> >  static void psi_avgs_work(struct work_struct *work);
> >
> > +static unsigned long psi_memtime_fixup(u32 growth);
> > +
> >  static void group_init(struct psi_group *group)
> >  {
> >       int cpu;
> > @@ -492,6 +494,21 @@ static u64 window_update(struct psi_window *win, u64 now, u64 value)
> >       return growth;
> >  }
> >
> > +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);
>
add vincent for advise on cpu load mechanism

> 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.
>
> Does util_avg capture that difference? I'm not confident it does - but
> correct me if I'm wrong. We need length of time during which and IRQ
> or an RT task preempted the old rq->curr, not absolute irq/rt length.
As far as my understanding, core's capacity = IRQ + DEADLINE + RT +
CFS. For a certain time period, all cfs tasks preempt each other
inside CFS's utilization. So the sleeping on refaults is counted in.
>
> (Btw, such preemption periods, in addition to being deducted from
> memory stalls, should probably also be added to CPU contention stalls,
> to make CPU pressure reporting more accurate as well.)