linux-kernel - Re: [question] sched: idle

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Date:	Tue, 10 Dec 2013 20:07:44 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Alex Shi <alex.shi@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [question] sched: idle_avg and migration latency

On 12/10/2013 04:20 PM, Alex Shi wrote:
> CC to MikeG, he written this part. :)
> I try to explain sth I know. I am sorry if my understanding incorrect.
>
> On 12/10/2013 07:30 PM, Daniel Lezcano wrote:
>>
>> Hi All,
>>
>> I am trying to understand how is computed the idle_avg and how it is
>> used regarding the migration latency.
>>
>> 1. What is the sysctl_sched_migration_cost value ? It is initialized to
>> 500000UL. Is it an arbitrarily chosen value ? Could it change depending
>> on the hardware performances ?
>
> current sysctl_sched_mirgration_cost is 0.5ms, used to limit
> overscheduling. Guess it is a kind of arbitrary. But it can be rewrite
> at /proc/sys/kernel/sched_migration_cost_ns.
> So if you find some new suitable value in particular scenario. guess
> PeterZ like to modify it. :)
>
>>
>>
>> 2. The idle_balance function checks:
>>
>>          if (this_rq->avg_idle < sysctl_sched_migration_cost)
>>                  return 0;
>>
>> IIUC, it is not worth to migrate a task to this cpu as we expect to run
>> another task before we can pull a task to the current cpu, right ?
>
> No, that used to prevent every idle_balance cause a task migration if
> idle balance happens too much and too quick, -- frequency more than task
> migration limitation.
>>
>> Then if there is no task to balance we will enter idle, thus we
>> initialize the idle_stamp to the current clock.
>
> If we pulled task, we will restart frequency calculation by set
> idle_stamp = 0;
> or if new task adding this rq, allow more idle_balance.

Thanks Alex for the explanation.

>> When another task is woken up with the ttwu_do_wakeup, the duration of
>> the idle time is computed in there:
>>
>>      if (rq->idle_stamp) {
>>          u64 delta = rq_clock(rq) - rq->idle_stamp;
>>          u64 max = 2*sysctl_sched_migration_cost;
>>
>>          if (delta > max)
>>              rq->avg_idle = max;
>>          else
>>              update_avg(&rq->avg_idle, delta);
>>          rq->idle_stamp = 0;
>>      }
>>
>> Why is the 'delta' leveraged by 'max' ?
>>
>>
>> 3. And finally the function update_avg does:
>>
>>      s64 diff = sample - *avg;
>>      *avg += diff >> 3;
>>
>> Why is diff >> 3 used instead of the number of values ?
>
> It is a kind of decay. but has no idea of why this value '3'. Guess
> MikeG has some reason.
>>
>> Thanks in advance for any answers
>>
>>    -- Daniel
>>
>
>


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