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

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Message-ID: <629937b1-6f97-41d1-aa4f-7349c2ffa29d@arm.com>
Date: Thu, 22 Aug 2024 13:07:57 +0100
From: Luis Machado <luis.machado@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Hongyan Xia <hongyan.xia2@....com>, 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

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.