linux-kernel - Re: [PATCH v3 3/7] sched: rotate the cpu search window for better spread

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Message-ID: <0dbdf3d0-8547-3290-0ad6-adf65d8aa976@oracle.com>
Date:   Fri, 28 Jun 2019 15:34:25 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, tglx@...utronix.de, steven.sistare@...cle.com,
        dhaval.giani@...cle.com, daniel.lezcano@...aro.org,
        vincent.guittot@...aro.org, viresh.kumar@...aro.org,
        tim.c.chen@...ux.intel.com, mgorman@...hsingularity.net
Subject: Re: [PATCH v3 3/7] sched: rotate the cpu search window for better
 spread


On 6/28/19 4:54 AM, Srikar Dronamraju wrote:
> * subhra mazumdar <subhra.mazumdar@...cle.com> [2019-06-26 18:29:15]:
>
>> Rotate the cpu search window for better spread of threads. This will ensure
>> an idle cpu will quickly be found if one exists.
> While rotating the cpu search window is good, not sure if this can find a
> idle cpu quickly. The probability of finding an idle cpu still should remain
> the same. No?
>
>> Signed-off-by: subhra mazumdar <subhra.mazumdar@...cle.com>
>> ---
>>   kernel/sched/fair.c | 10 ++++++++--
>>   1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> @@ -6219,9 +6219,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>>   		}
>>   	}
>>   
>> +	if (per_cpu(next_cpu, target) != -1)
>> +		target_tmp = per_cpu(next_cpu, target);
>> +	else
>> +		target_tmp = target;
>> +
>>   	time = local_clock();
>>   
>> -	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
>> +	for_each_cpu_wrap(cpu, sched_domain_span(sd), target_tmp) {
>> +		per_cpu(next_cpu, target) = cpu;
> Shouldn't this assignment be outside the for loop.
> With the current code,
> 1. We keep reassigning multiple times.
> 2. The last assignment happes for idle_cpu and sometimes the
> assignment is for non-idle cpu.
We want the last assignment irrespective of it was an idle cpu or not since
in both cases we want to track the boundary of search.

Thanks,
Subhra