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SubjectRe: [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@huawei.com> wrote:
>
> On 2021/12/22 18:47, Vincent Guittot wrote:
> > On Sat, 11 Dec 2021 at 11:43, Yicong Yang <yangyicong@hisilicon.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@hisilicon.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
> >>
> > .
> >

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