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

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Date:   Wed, 14 Feb 2018 10:11:21 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Rohit Jain <rohit.k.jain@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        joelaf@...gle.com, vincent.guittot@...aro.org,
        morten.rasmussen@....com, eas-dev@...ts.linaro.org
Subject: Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in
 select_idle_sibling

On 02/09/2018 11:05 PM, Rohit Jain wrote:
> 
> On 02/09/2018 07:46 AM, Dietmar Eggemann wrote:
>> On 02/09/2018 01:53 PM, Peter Zijlstra wrote:
>>> On Mon, Jan 29, 2018 at 03:27:09PM -0800, Rohit Jain wrote:
>>
>> [...]
>>
>>>> @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct 
>>>> *p, struct sched_domain *sd, int t
>>>>               return -1;
>>>>           if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>>>>               continue;
>>>> +        if (idle_cpu(cpu)) {
>>>> +            if (full_capacity(cpu)) {
>>>> +                best_cpu = cpu;
>>>> +                break;
>>>> +            } else if (capacity_of(cpu) > best_cap) {
>>>> +                best_cap = capacity_of(cpu);
>>>> +                best_cpu = cpu;
>>>> +            }
>>>> +        }
>>>
>>> No need for the else. And you'll note you're once again inconsistent
>>> with your previous self.
>>>
>>> But here I worry about big.little a wee bit. I think we're allowed big
>>> and little cores on the same L3 these days, and you can't directly
>>> compare capacity between them.
>>>
>>> Morten / Dietmar, any comments?
>>
>> Yes, for DynamIQ (big.little successor) systems, those cpus can have 
>> different capacity_orig_of() values already.
>>
> 
> OK, given that there are asymmetric capacities in L3 cores, we would
> probably have something like the below(?) in select_idle_cpu:
> 
>            if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>                continue;
> +        if (idle_cpu(cpu) && !reduced_capacity(cpu))
> +            break;
> 

Only returning the first idle cpu w/o reduced capacity will definitely 
work better here on future DynamIQ systems than trying to find the 
best_cap cpu by taking capacity_of() into consideration.