Re: [PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle_cpu

[Vincent Guittot]

On Thu, 6 Jan 2022 at 10:10, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>
> On 2022/1/4 22:18, Vincent Guittot wrote:
> > On Thu, 23 Dec 2021 at 09:23, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
> >>
> >> On 2021/12/22 18:47, Vincent Guittot wrote:
> >>> On Sat, 11 Dec 2021 at 11:43, Yicong Yang <yangyicong@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >>>> ---
> >>>>  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
> >
>
> uh I misunderstood. not sure I've missed something, but looks like we also have this problem
> when updating &this_sd->avg_scan_cost?

No because this_sd->avg_scan_cost is only used by the local cpu so you
don't have several cpus trying to update it simultaneously.

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