On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi <patrick.bell...@arm.com> wrote: > > On 28-Nov 11:02, Peter Zijlstra wrote: > > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote: > > > > > Is there anything else that I should do for these patches ? > > > > IIRC, Morten mention they break util_est; Patrick was going to explain. > > I guess the problem is that, once we cross the current capacity, > strictly speaking util_avg does not represent anymore a utilization. > > With the new signal this could happen and we end up storing estimated > utilization samples which will overestimate the task requirements. > > We will have a spike in estimated utilization at next wakeup, since we > use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in > case we collect multiple samples above the current capacity.
TBH I don't see how it's different from current implementation with a task that was scheduled on big core and now wakes up on little core. The util_est is overestimated as well. But I'm fine with adding your proposal on to on the patchset > > So, a possible fix could be to avoid storing util_est samples if we > end up with a utilization above the current capacity. > > Something like: > > ----8<--- > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index ac855b2f4774..93e0cf5d8a76 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3661,6 +3661,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct > task_struct *p, bool task_sleep) > if (!task_sleep) > return; > > + /* Skip samples which do not represent an actual utilization */ > + if (unlikely(task_util(p) > capacity_of(task_cpu(p)))) > + return; > + > /* > * If the PELT values haven't changed since enqueue time, > * skip the util_est update. > ---8<--- > > Could that work ? > > Maybe using a new utility function to wrap the new check. > > -- > #include <best/regards.h> > > Patrick Bellasi