linux-kernel - Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in select_idle

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Date:   Tue, 10 Oct 2017 11:02:59 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Atish Patra <atish.patra@...cle.com>, linux-kernel@...r.kernel.org,
        eas-dev@...ts.linaro.org
Cc:     peterz@...radead.org, mingo@...hat.com, joelaf@...gle.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        morten.rasmussen@....com
Subject: Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in
 select_idle_sibling code path

Hi Atish,

Thanks for the comments

On 10/10/2017 08:54 AM, Atish Patra wrote:
> <snip>
>>
>> Signed-off-by: Rohit Jain <rohit.k.jain@...cle.com>
>> ---
>>   kernel/sched/fair.c | 37 ++++++++++++++++++++++++++++---------
>>   1 file changed, 28 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index eaede50..5b1f7b9 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6004,7 +6004,7 @@ 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))
> Do we need to skip the entire core just because 1st cpu in the core 
> doesn't have full capacity ?
> Let's say that is the only idle core available. It will go and try to 
> select_idle_cpu() to find the idlest cpu.
> Is it worth spending extra time to search an idle cpu with full 
> capacity when there are idle cores available ?

This has been previously discussed:
https://lkml.org/lkml/2017/10/3/1001

Returning the best CPU within the idle core did not result in a
statistically significant performance benefit, hence I went with Joel's
suggestion to keep the code simple.

Thanks,
Rohit

<snip>