linux-kernel - Re: [PATCH v2 1/2 RESEND] sched/fair: take into account scheduling domain in select_idle

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Date: Wed, 10 Jan 2024 18:57:24 +0100
From: Keisuke Nishimura <keisuke.nishimura@...ia.fr>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 linux-kernel@...r.kernel.org, Dietmar Eggemann <dietmar.eggemann@....com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Julia Lawall <julia.lawall@...ia.fr>, Xunlei Pang
 <xlpang@...ux.alibaba.com>, Abel Wu <wuyun.abel@...edance.com>
Subject: Re: [PATCH v2 1/2 RESEND] sched/fair: take into account scheduling
 domain in select_idle_smt()

>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 533547e3c90a..66457d4b8965 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -7311,13 +7311,19 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
>>   /*
>>    * Scan the local SMT mask for idle CPUs.
>>    */
>> -static int select_idle_smt(struct task_struct *p, int target)
>> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
>>   {
>>          int cpu;
>>
>>          for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
>>                  if (cpu == target)
>>                          continue;
>> +               /*
>> +                * Check if the CPU is in the LLC scheduling domain of @target.
>> +                * Due to isolcpus, there is no guarantee that all the siblings are in the domain.
>> +                */
>> +               if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> 
> commit df3cb4ea1fb6 ("sched/fair: Fix wrong cpu selecting from isolated domain")
> also checked if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
> 
> Why didn't you also re-add this test ?
> 
> 

Thank you for your comment. This is because the iterator "for_each_cpu_and(cpu, 
cpu_smt_mask(target), p->cpus_ptr)" ensures that cpu is included in p->cpus_ptr.

The iterator has changed. FYI, here is the change made in commit df3cb4ea1fb6:

         for_each_cpu(cpu, cpu_smt_mask(target)) {
-               if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+               if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
+                   !cpumask_test_cpu(cpu, sched_domain_span(sd)))
                         continue;
                 if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))

best,
Keisuke