Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 4 Jan 2022 15:18:03 +0100 | Subject | Re: [PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle_cpu |
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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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