Re: [PATCH v2 08/11] sched: get CPU's activity statistic

[Vincent Guittot <vincent.guittot@linaro.org>]

On 3 June 2014 17:50, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, May 28, 2014 at 04:47:03PM +0100, Morten Rasmussen wrote:
>> Since we may do periodic load-balance every 10 ms or so, we will perform
>> a number of load-balances where runnable_avg_sum will mostly be
>> reflecting the state of the world before a change (new task queued or
>> moved a task to a different cpu). If you had have two tasks continuously
>> on one cpu and your other cpu is idle, and you move one of the tasks to
>> the other cpu, runnable_avg_sum will remain unchanged, 47742, on the
>> first cpu while it starts from 0 on the other one. 10 ms later it will
>> have increased a bit, 32 ms later it will be 47742/2, and 345 ms later
>> it reaches 47742. In the mean time the cpu doesn't appear fully utilized
>> and we might decide to put more tasks on it because we don't know if
>> runnable_avg_sum represents a partially utilized cpu (for example a 50%
>> task) or if it will continue to rise and eventually get to 47742.
>
> Ah, no, since we track per task, and update the per-cpu ones when we
> migrate tasks, the per-cpu values should be instantly updated.
>
> If we were to increase per task storage, we might as well also track
> running_avg not only runnable_avg.

I agree that the removed running_avg should give more useful
information about the the load of a CPU.

The main issue with running_avg is that it's disturbed by other tasks
(as point out previously). As a typical example,  if we have 2 tasks
with a load of 25% on 1 CPU, the unweighted runnable_load_avg will be
in the range of [100% - 50%] depending of the parallelism of the
runtime of the tasks whereas the reality is 50% and the use of
running_avg will return this value

Vincent