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

[Morten Rasmussen]

On Wed, Jun 04, 2014 at 12:07:29PM +0100, Vincent Guittot wrote:
> On 4 June 2014 12:36, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> > On Wed, Jun 04, 2014 at 11:17:24AM +0100, Peter Zijlstra wrote:
> >> On Wed, Jun 04, 2014 at 11:32:10AM +0200, Vincent Guittot wrote:
> >> > On 4 June 2014 10:08, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > > On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote:
> >> > >> On 3 June 2014 17:50, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
> >>
> >> Let me explain the 75%, take any one of the above scenarios. Lets call
> >> the two tasks A and B, and let for a moment assume A always wins and
> >> runs first, and then B.
> >>
> >> So A will be runnable for 25%, B otoh will be runnable the entire time A
> >> is actually running plus its own running time, giving 50%. Together that
> >> makes 75%.
> >>
> >> If you release the assumption that A runs first, but instead assume they
> >> equally win the first execution, you get them averaging at 37.5% each,
> >> which combined will still give 75%.
> >
> > But that is assuming that the first task gets to run to completion of it
> > busy period. If it uses up its sched_slice and we switch to the other
> > tasks, they both get to wait.
> >
> > For example, if the sched_slice is 5 ms and the busy period is 10 ms,
> > the execution pattern would be: A, B, A, B, idle, ... In that case A is
> > runnable for 15 ms and B is for 20 ms. Assuming that the overall period
> > is 40 ms, the A runnable is 37.5% and B is 50%.
> 
> The exact value for your scheduling example above is:
> A runnable will be 47% and B runnable will be 60% (unless i make a
> mistake in my computation)

I get:

A: 15/40 ms = 37.5%
B: 20/40 ms = 50%

Schedule:

   | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms |
A:   run     rq     run  ----------- sleeping -------------  run
B:   rq      run    rq    run   ---- sleeping -------------  rq

> and CPU runnable will be 60% too

rq->avg.runnable_avg_sum should be 50%. You have two tasks running for
20 ms every 40 ms.

Right?

Morten
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