linux-kernel - Re: slowdown due to reader-owned rwsem time-based spinning

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Date:   Mon, 19 Oct 2020 21:48:07 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Waiman Long <longman@...hat.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Gilles Muller <Gilles.Muller@...ia.fr>
Subject: Re: slowdown due to reader-owned rwsem time-based spinning



On Mon, 19 Oct 2020, Waiman Long wrote:

> On 10/15/20 7:38 AM, Julia Lawall wrote:
> > Hello,
> >
> > Phoenix is an implementation of map reduce:
> >
> > https://github.com/kozyraki/phoenix
> >
> > The phoenix-2.0/tests subdirectory contains some benchmarks, including
> > word_count.
> >
> > At the same time, on my server, since v5.8, the kernel has changed from
> > using the governor intel_pstate by default to using intel_cpufreq.
> > Intel_cpufreq causes kworkers to run on all cores every 0.004 seconds,
> > while intel_pstate involves very few such stray processes.
> >
> > Suprisingly, all those kworkers cause the word_count benchmark to run 2-3
> > times faster.  I bisected the problem back to the following commit, whcih
> > was introduced in v5.3:
> >
> > commit 7d43f1ce9dd075d8b2aa3ad1f3970ef386a5c358
> > Author: Waiman Long <longman@...hat.com>
> > Date:   Mon May 20 16:59:13 2019 -0400
> >
> >      locking/rwsem: Enable time-based spinning on reader-owned rwsem
> >
> > Representative traces are attached.  word_count_5.9pwrsvpassive_1.pdf is
> > the one with the kworkers.
> >
> > I don't know the Phoenix code in detail, but the problem seems to be in
> > the infrastructure not the specific word count aplication, because most of
> > the benchmarks seem to suffer similarly.  Some of the other benchmarks
> > seem to take a variable and long amount of time to get started in the
> > active mode, so perhaps the problem could be in reading the initial
> > dataset.
> >
> > Before I plunge into it, do you have any suggestions as to what could be
> > the problem?
>
> I am a bit confused as to what you are looking for. So you said this patch
> make the benchmark run 2-3 times faster. Is this a problem? What are you
> trying to achieve? Is it to make the passive case similar to the active case?

Sorry, it seems that I was not clear.  Prior to the commit above the
active case had good performance,  The patch caused the active case to
slow down by 2-3 times.  Adding lots of kworkers that interrupt the
threads eliminated the slowdown.

>
> What this patch does is to allow writer waiting for a rwsem to spin for a
> while hoping the readers will release the lock soon to acquire the lock.
> Before that, the writer will go to sleep immediately when the rwsem is owned
> by readers. Probably because of that, the kworkers keep on running for a much
> longer time as long as there are no other tasks competing for the CPUs.

No, the kworkers don't run for a long time.  My hypothesis is that the
kworkers interrupt a thread that is spinning waiting for a lock and thus
allow the thread that is holding the lock to run.

julia