Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling

[Joel Fernandes]

On Tue, Jan 30, 2018 at 11:47 AM, Rohit Jain <rohit.k.jain@xxxxxxxxxx> 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