linux-kernel - Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle

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Date:   Mon, 5 Feb 2018 22:42:29 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     Rohit Jain <rohit.k.jain@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, steven.sistare@...cle.com,
        dhaval.giani@...cle.com,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        "Cc: EAS Dev" <eas-dev@...ts.linaro.org>
Subject: Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling

On Tue, Jan 30, 2018 at 11:47 AM, Rohit Jain <rohit.k.jain@...cle.com> wrote:
[...]
>>> @@ -6102,7 +6107,8 @@ static int select_idle_core(struct task_struct *p,
>>> struct sched_domain *sd, int
>>>    */
>>>   static int select_idle_smt(struct task_struct *p, struct sched_domain
>>> *sd, int target)
>>>   {
>>> -       int cpu;
>>> +       int cpu, rcpu = -1;
>>> +       unsigned long max_cap = 0;
>>>
>>>          if (!static_branch_likely(&sched_smt_present))
>>>                  return -1;
>>> @@ -6110,11 +6116,13 @@ static int select_idle_smt(struct task_struct *p,
>>> struct sched_domain *sd, int t
>>>          for_each_cpu(cpu, cpu_smt_mask(target)) {
>>>                  if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>>>                          continue;
>>> -               if (idle_cpu(cpu))
>>> -                       return cpu;
>>> +               if (idle_cpu(cpu) && (capacity_of(cpu) > max_cap)) {
>>> +                       max_cap = capacity_of(cpu);
>>> +                       rcpu = cpu;
>>
>> At the SMT level, do you need to bother with choosing best capacity
>> among threads? If RT is eating into one of the SMT thread's underlying
>> capacity, it would eat into the other's. Wondering what's the benefit
>> of doing this here.
>
>
> Yes, you are right because of SD_SHARE_CPUCAPACITY, however the benefit
> is that if don't do this check, we might end up picking a SMT thread
> which has "high" RT/IRQ activity and be on the run queue for a while,
> till the pull side can bail us out.

Do your tests show a difference in results though with such change
(for select_idle_smt)?

thanks,

- Joel