Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Track target domain's avg_scan_cost in select_idle_cpu | From | Yicong Yang <> | Date | Thu, 6 Jan 2022 17:10:07 +0800 |
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On 2022/1/4 22:18, Vincent Guittot wrote: > 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 >
uh I misunderstood. not sure I've missed something, but looks like we also have this problem when updating &this_sd->avg_scan_cost?
>> >>>> } >>>> >>>> return idle_cpu; >>>> -- >>>> 2.33.0 >>>> >>> . >>> > . >
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