linux-kernel - Re: [PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle

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Message-ID: <CAKfTPtCPRk+2L6wSHBpN-ewTX1Cj4U7a_m1itsSk9Yav-jE-9Q@mail.gmail.com>
Date:   Tue, 4 Jan 2022 15:18:03 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Yicong Yang <yangyicong@...wei.com>
Cc:     Yicong Yang <yangyicong@...ilicon.com>, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        bristot@...hat.com, prime.zeng@...wei.com, linuxarm@...wei.com,
        21cnbao@...il.com
Subject: Re: [PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle_cpu

On Thu, 23 Dec 2021 at 09:23, Yicong Yang <yangyicong@...wei.com> wrote:
>
> On 2021/12/22 18:47, Vincent Guittot wrote:
> > On Sat, 11 Dec 2021 at 11:43, Yicong Yang <yangyicong@...ilicon.com> wrote:
> >>
> >> We regulate the LLC domain scan in select_idle_cpu() by comparing
> >> the average scan cost of this_sd against the average idle time of
> >> this_rq. This is correct when the domain to scan is the LLC domain
> >> of this cpu. But when the domain to scan is different from this
> >> LLC domain, we'll have an inaccurate estimation of the scan cost
> >> on the target domain as this_sd->avg_scan_cost contains contributions
> >> of scanning other domains besides the target domain.
> >>
> >> Track the avg_scan_cost of the target domain to make the estimation
> >> more accurate.
> >>
> >> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> >> ---
> >>  kernel/sched/fair.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 6e476f6d9435..6301740d98cb 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -6267,7 +6267,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>                 }
> >>
> >>                 avg_idle = this_rq->wake_avg_idle;
> >> -               avg_cost = this_sd->avg_scan_cost + 1;
> >> +               avg_cost = sd->avg_scan_cost + 1;
> >>
> >>                 span_avg = sd->span_weight * avg_idle;
> >>                 if (span_avg > 4*avg_cost)
> >> @@ -6305,7 +6305,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>                  */
> >>                 this_rq->wake_avg_idle -= min(this_rq->wake_avg_idle, time);
> >>
> >> -               update_avg(&this_sd->avg_scan_cost, time);
> >> +               update_avg(&sd->avg_scan_cost, time);
> >
> > But then you can have several cpus updating the same value simultaneously
> >
>
> yes. sd->avg_scan_cost should includes the contributions of all the cpus scanned the sd.
>
> We regulated the scanning nr based on two things:
> - avg_idle: to indicate how much time we can have for this time scanning
> - avg_cost: to indicate how much time we'll spend for scanning the target domain based
>             on the history cost
>
> Previously sd->avg_scan_cost may not reflect the cost as it count the scanning cost
> on the domain of the scanner cpu, which may not be the domain the cpu scanned.
> For example, cpu 0 on llc A scanned llc B and llc C, we'll count the cost of scanning B
> and C on llc A's avg_scan_cost and we'll use this to estimate the cost for scanning
> llc A, which is not accurate.

I mean that you can now have several CPUs that will read, modify,
write sd->avg_scan_cost simultaneously without any protection

>
> >>         }
> >>
> >>         return idle_cpu;
> >> --
> >> 2.33.0
> >>
> > .
> >