Re: [question] sched: idle_avg and migration latency

[Daniel Lezcano]

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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