Re: [question] sched: idle_avg and migration latency

[Mike Galbraith <bitbucket@online.de>]

On Tue, 2013-12-10 at 12:30 +0100, 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 ?

Yeah, it's a magic number.  We used to use boot time measurements.

> 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's all about not beating living hell outta ourselves on every
micro-idle.  As with all load balancing, it's usually too much balancing
that creates a problem.  You need it, but it's really expensive, so less
is more.

> Then if there is no task to balance we will enter idle, thus we 
> initialize the idle_stamp to the current clock.
> 
> 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' ?

That has changed a little recently.  I originally slammed avg_idle
itself straight to max to ensure that a bursty load would idle balance,
and not use stale data.  If you start cross core switching at high
frequency, you'll still shut idle balancing quickly.

> 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 ?

Ingo's quick like bunny smooth average.

-Mike