Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Paul McKenney <paulmck@kernel.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ
Date: Mon, 25 Jan 2021 18:53:45 +0100	[thread overview]
Message-ID: <8db5ebd3-4e5c-cd7e-e0cb-dc86c3cafb5c@arm.com> (raw)
In-Reply-To: <CAKfTPtA7Fte3WWw1=-BaNX-fpfEeWicUOEgfzJsgCTcygzZTQA@mail.gmail.com>

On 25/01/2021 18:30, Vincent Guittot wrote:
> On Mon, 25 Jan 2021 at 11:45, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>
>> On 22/01/2021 20:10, Joel Fernandes wrote:
>>> Hi Vincent,
>>>
>>> Thanks for reply. Please see the replies below:
>>>
>>> On Fri, Jan 22, 2021 at 05:56:22PM +0100, Vincent Guittot wrote:
>>>> On Fri, 22 Jan 2021 at 16:46, Joel Fernandes (Google)
>>>> <joel@joelfernandes.org> wrote:

[...]

>> If I understood you correctly, you want to avoid these frequent calls
>> to update_blocked_averages() here to further avoid invoking sched_util
>> via update_blocked_averages() -> cpufreq_update_util() (since 'decayed'
>> is set) very often in your setup.
> 
> So It's not clear if the problem that joel wants to raise, is about:
> - the running time of  update_blocked_averages
> - the running time of the cpufreq_update_util which is called because
> utilization has decayed during the update of blocked load
> - the wake up latency because of newly_idle lb

Pretty much so.

IIRC his interest is driven by the fact that he saw much less activity
in newly_idle lb and therefore cpufreq_update_util on a system with the
same kernel and userspace but with less CPUs (i.e. also smaller
frequency domains) and less cgroups (with blocked load) and started
wondering why.

I assume that since he understands this environment now much better, he
should be able to come up with better test numbers to show if there is a
performance issue on his 2+6 DynamIQ system and if yes, where exactly in
this code path.