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