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Date:   Tue, 21 Nov 2023 23:01:50 +0000
From:   Qais Yousef <qyousef@layalina.io>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        rafael@kernel.org, viresh.kumar@linaro.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        lukasz.luba@arm.com
Subject: Re: [PATCH] sched/pelt: avoid underestimate of task utilization
Message-ID: <20231121230150.eq2kc72bvyn6ltur@airbuntu>
References: <20231122140119.472110-1-vincent.guittot@linaro.org>
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On 11/22/23 15:01, Vincent Guittot wrote:
> It has been reported that thread's util_est can significantly decrease as
> a result of sharing the CPU with other threads. The use case can be easily
> reproduced with a periodic task TA that runs 1ms and sleeps 100us.
> When the task is alone on the CPU, its max utilization and its util_est is
> around 888. If another similar task starts to run on the same CPU, TA will
> have to share the CPU runtime and its maximum utilization will decrease
> around half the CPU capacity (512) then TA's util_est will follow this new
> maximum trend which is only the result of sharing the CPU with others
> tasks. Such situation can be detected with runnable_avg wich is close or
> equal to util_avg when TA is alone but increases above util_avg when TA
> shares the CPU with other threads and wait on the runqueue.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---

So effectively if have two always running tasks on the same CPU their util_est
will converge to 1024 instead of 512 now, right?

I guess this is more accurate, yes. And I think this will hit the same
limitation we can hit with uclamp_max. If for example there are two tasks that
are 650 if they run alone, they would appear as 1024 now (instead of 512) if
they share the CPU because combined running there will be no idle time on the
CPU and appear like always running tasks, I think.

If I got it right, I think this should not be a problem in practice because the
only reason these two tasks will be stuck on the same CPU is because the load
balancer can't do anything about it to spread them; which indicates the system
must be busy anyway. Once there's more idle time elsewhere, they should be
spread and converge to 650 again.

Not my forte, but LGTM anyway.


Cheers

--
Qais Yousef