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

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Date:   Mon, 5 Feb 2018 22:50:41 -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 Wed, Jan 31, 2018 at 9:50 AM, Rohit Jain <rohit.k.jain@...cle.com> wrote:
>>>>>    kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++----------
>>>>>    1 file changed, 28 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 26a71eb..ce5ccf8 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu)
>>>>>           return cpu_rq(cpu)->cpu_capacity_orig;
>>>>>    }
>>>>>
>>>>> +static inline bool full_capacity(int cpu)
>>>>> +{
>>>>> +       return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
>>>>> +}
>>>>> +
>>>>>    static unsigned long cpu_avg_load_per_task(int cpu)
>>>>>    {
>>>>>           struct rq *rq = cpu_rq(cpu);
>>>>> @@ -6081,7 +6086,7 @@ static int select_idle_core(struct task_struct
>>>>> *p,
>>>>> struct sched_domain *sd, int
>>>>>
>>>>>                   for_each_cpu(cpu, cpu_smt_mask(core)) {
>>>>>                           cpumask_clear_cpu(cpu, cpus);
>>>>> -                       if (!idle_cpu(cpu))
>>>>> +                       if (!idle_cpu(cpu) || !full_capacity(cpu))
>>>>>                                   idle = false;
>>>>>                   }
>>>>
>>>> There's some difference in logic between select_idle_core and
>>>> select_idle_cpu as far as the full_capacity stuff you're adding goes.
>>>> In select_idle_core, if all CPUs are !full_capacity, you're returning
>>>> -1. But in select_idle_cpu you're returning the best idle CPU that's
>>>> the most cap among the !full_capacity ones. Why there is this
>>>> different in logic? Did I miss something?
>>>>
>
> <snip>
>
>> Dude :) That is hardly an answer to the question I asked. Hint:
>> *different in logic*.
>
>
> Let me re-try :)
>
> For select_idle_core, we are doing a search for a fully idle and full
> capacity core, the fail-safe is select_idle_cpu because we will re-scan
> the CPUs. The notion is to select an idle CPU no matter what, because
> being on an idle CPU is better than waiting on a non-idle one. In
> select_idle_core you can be slightly picky about the core because
> select_idle_cpu is a fail safe. I measured the performance impact of
> choosing the "best among low cap" vs the code changes I have (for
> select_idle_core) and could not find a statistically significant impact,
> hence went with the simpler code changes.

That's Ok with me. Just that I remember Peter messing with this path
and that it was expensive to scan too much for some systems. The other
thing is you're really doing to do a "fail safe" as you call it search
here with SIS_PROP set. Do you see a difference in perf when doing the
same approach as you took in select_idle_core?

Peter, are you with the approach Rohit has adopted to pick best
capacity idle CPU in select_idle_cpu? I guess nr--; will bail out
early if we have SIS_PROP set, incase the scan cost gets too much but
then again we might end scanning too few CPUs.

thanks,

- Joel