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

[Morten Rasmussen]

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%.

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