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
