Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

[Hongyan Xia]

On 22/08/2024 15:58, Vincent Guittot wrote:
> On Thu, 22 Aug 2024 at 14:10, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> > On Thu, 22 Aug 2024 at 14:08, Luis Machado <luis.machado@xxxxxxx> wrote:
> > > Vincent,
> > >
> > > On 8/22/24 11:28, Luis Machado wrote:
> > > > On 8/22/24 10:53, Vincent Guittot wrote:
> > > > > On Thu, 22 Aug 2024 at 11:22, Luis Machado <luis.machado@xxxxxxx> wrote:
> > > > > > On 8/22/24 09:19, Vincent Guittot wrote:
> > > > > > > Hi,
> > > > > > >
> > > > > > > On Wed, 21 Aug 2024 at 15:34, Hongyan Xia <hongyan.xia2@xxxxxxx> wrote:
> > > > > > > > Hi Peter,
> > > > > > > >
> > > > > > > > Sorry for bombarding this thread in the last couple of days. I'm seeing
> > > > > > > > several issues in the latest tip/sched/core after these patches landed.
> > > > > > > >
> > > > > > > > What I'm now seeing seems to be an unbalanced call of util_est. First, I applied
> > > > > > >
> > > > > > > I also see a remaining util_est for idle rq because of an unbalance
> > > > > > > call of util_est_enqueue|dequeue
> > > > > >
> > > > > > I can confirm issues with the utilization values and frequencies being driven
> > > > > > seemingly incorrectly, in particular for little cores.
> > > > > >
> > > > > > What I'm seeing with the stock series is high utilization values for some tasks
> > > > > > and little cores having their frequencies maxed out for extended periods of
> > > > > > time. Sometimes for 5+ or 10+ seconds, which is excessive as the cores are mostly
> > > > > > idle. But whenever certain tasks get scheduled there, they have a very high util
> > > > > > level and so the frequency is kept at max.
> > > > > >
> > > > > > As a consequence this drives up power usage.
> > > > > >
> > > > > > I gave Hongyan's draft fix a try and observed a much more reasonable behavior for
> > > > > > the util numbers and frequencies being used for the little cores. With his fix,
> > > > > > I can also see lower energy use for my specific benchmark.
> > > > >
> > > > > The main problem is that the util_est of a delayed dequeued task
> > > > > remains on the rq and keeps the rq utilization high and as a result
> > > > > the frequency higher than needed.
> > > > >
> > > > > The below seems to works for me and keep sync the enqueue/dequeue of
> > > > > uti_test with the enqueue/dequeue of the task as if de dequeue was not
> > > > > delayed
> > > > >
> > > > > Another interest is that we will not try to migrate a delayed dequeue
> > > > > sleeping task that doesn't actually impact the current load of the cpu
> > > > > and as a result will not help in the load balance. I haven't yet fully
> > > > > checked what would happen with hotplug
> > > >
> > > > Thanks. Those are good points. Let me go and try your patch.
> > >
> > > I gave your fix a try, but it seems to make things worse. It is comparable
> > > to the behavior we had before Peter added the uclamp imbalance fix, so I
> > > believe there is something incorrect there.
> >
> > we need to filter case where task are enqueued/dequeued several
> > consecutive times. That's what I'm look now
>
> I just realize before that It's not only util_est but the h_nr_running
> that keeps delayed tasks as well so all stats of the rq are biased:
> h_nr_running, util_est, runnable avg and load_avg.

After staring at the code even more, I think the situation is worse.

First thing is that uclamp might also want to be part of these stats 
(h_nr_running, util_est, runnable_avg, load_avg) that do not follow 
delayed dequeue which needs to be specially handled in the same way. The 
current way of handling uclamp in core.c misses the frequency update, 
like I commented before.

Second, there is also an out-of-sync issue in update_load_avg(). We only 
update the task-level se in delayed dequeue and requeue, but we return 
early and the upper levels are completely skipped, as if the delayed 
task is still on rq. This de-sync is wrong.