linux-kernel - [question] sched: idle_avg and migration latency

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Message-ID: <52A6FB5C.7010706@linaro.org>
Date:	Tue, 10 Dec 2013 12:30:36 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	Alex Shi <alex.shi@...aro.org>
Subject: [question] sched: idle_avg and migration latency

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 ?

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 ?

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' ?

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 ?

Thanks in advance for any answers

   -- Daniel

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