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From: Joel Fernandes <joel@joelfernandes.org>
To: Beata Michalska <beata.michalska@arm.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Quentin Perret <qperret@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Qais Yousef <qais.yousef@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>
Subject: iowait boost is broken
Date: Mon, 7 Jun 2021 12:19:01 -0400	[thread overview]
Message-ID: <CAEXW_YTcO=hbmdq3nOx2RJfT2yPyoFnQx5niB38R2Lzpsp38bA@mail.gmail.com> (raw)

Hi all,
Looks like iowait boost is completely broken upstream. Just
documenting my findings of iowait boost issues:

1. If a CPU requests iowait boost in a cluster, another CPU can go
ahead and reset very quickly it since it thinks there's no new request
for the iowait boosting CPU
2. If the iowait is longer than a tick, then successive iowait boost
doubling does not happen. So heavy I/O waiting code never gets a
boost.
3. update_load_avg() is triggered right after the the iowait boost
request which makes another cpufreq update request, this request is a
non-iowait boost one so it ends up resetting the iowait boost request
(in the same path!).
4. Same as #3 but due the update_blocked_averages from new idle balance path.

Here is a patch that tries to address these problems and I see better
cpufreq boosting happening, however it is just a test broken patch to
highlight the issues:
https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=sched/5.4/iowait-boost-debug-1&id=3627d896d499d168fef9a388e5d6b3359acc3423

I think we ought to rewrite the whole mess instead of fixing it since
a lot has changed in scheduler code over time it feels. Beata is
working on rewriting the whole iowait boost infra, I am glad she has
started the work on this and looking forward to helping with the
patches.

thanks,
 - Joel

             reply	other threads:[~2021-06-07 16:24 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-07 16:19 Joel Fernandes [this message]
2021-06-07 19:10 ` iowait boost is broken Beata Michalska
2021-06-08  9:26   ` Qais Yousef
2021-06-08 17:46   ` Peter Zijlstra
2021-06-09  9:50     ` Quentin Perret
2021-06-10 15:28       ` Qais Yousef
2021-06-10 13:33     ` Beata Michalska
2021-06-08 19:09   ` Joel Fernandes
2021-06-10 13:30     ` Beata Michalska
2021-06-10 18:52       ` Joel Fernandes

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