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Date:   Fri, 5 Jun 2020 22:33:47 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc:     Alexander Monakov <amonakov@ispras.ru>,
        linux-kernel@vger.kernel.org, Linux PM <linux-pm@vger.kernel.org>,
        Giovanni Gherdovich <ggherdovich@suse.cz>, qperret@google.com,
        juri.lelli@redhat.com,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Doug Smythies <dsmythies@telus.net>
Subject: Re: schedutil issue with serial workloads
Message-ID: <20200605203347.GM3976@hirez.programming.kicks-ass.net>
References: <alpine.LNX.2.20.13.2006042341160.3984@monopod.intra.ispras.ru>
 <c3145e26-56c8-4979-513c-cfac191e989b@intel.com>
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On Fri, Jun 05, 2020 at 06:51:12PM +0200, Rafael J. Wysocki wrote:
> On 6/4/2020 11:29 PM, Alexander Monakov wrote:

> > this is a question/bugreport about behavior of schedutil on serial workloads
> > such as rsync, or './configure', or 'make install'. These workloads are
> > such that there's no single task that takes a substantial portion of CPU
> > time, but at any moment there's at least one runnable task, and overall
> > the workload is compute-bound. To run the workload efficiently, cpufreq
> > governor should select a high frequency.
> > 
> > Assume the system is idle except for the workload in question.
> > 
> > Sadly, schedutil will select the lowest frequency, unless the workload is
> > confined to one core with taskset (in which case it will select the
> > highest frequency, correctly though somewhat paradoxically).
> 
> That's because the CPU utilization generated by the workload on all CPUs is
> small.
> 
> Confining it to one CPU causes the utilization of this one to grow and so
> schedutil selects a higher frequency for it.

My initial question was why doesn't io-boosting fix this up, but a quick
look at our pipe code shows me that it doesn't seem to use
io_schedule().

That is currently our only means to express 'someone is waiting on us'
to which we then say 'lets hurry up a bit'.

Because, as you've found, if the tasks do not queue up, there is nothing
to push the frequency up.