Messages in this thread Patch in this message | | | Date | Wed, 28 Nov 2018 11:53:36 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT |
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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.
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
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