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

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Message-ID: <aa81d37e-ad9c-42c6-a104-fe8496c5d907@arm.com>
Date: Thu, 29 Aug 2024 16:42:22 +0100
From: Hongyan Xia <hongyan.xia2@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>,
 Luis Machado <luis.machado@....com>
Cc: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
 juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com,
 linux-kernel@...r.kernel.org, kprateek.nayak@....com,
 wuyun.abel@...edance.com, youssefesmat@...omium.org, tglx@...utronix.de,
 efault@....de
Subject: Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

On 22/08/2024 15:58, Vincent Guittot wrote:
> On Thu, 22 Aug 2024 at 14:10, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>>
>> On Thu, 22 Aug 2024 at 14:08, Luis Machado <luis.machado@....com> 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@....com> wrote:
>>>>>>
>>>>>> On 8/22/24 09:19, Vincent Guittot wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On Wed, 21 Aug 2024 at 15:34, Hongyan Xia <hongyan.xia2@....com> 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.