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

[Peter Zijlstra <peterz@infradead.org>]

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On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote:
> 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

I'm not sure I see how 100% is possible, but yes I agree that runnable
can indeed be inflated due to this queueing effect.

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