linux-kernel - Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll

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Date:   Mon, 26 Mar 2018 12:32:12 -0400
From:   Rik van Riel <riel@...riel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

On Sun, 2018-03-25 at 23:34 +0200, Rafael J. Wysocki wrote:
> On Sunday, March 25, 2018 10:15:52 PM CEST Rik van Riel wrote:
> > 
> > I plan to try two more things:
> > 
> > 1) Disable polling on SMT systems, with
> >    the idea that putting one thread to
> >    sleep with monitor/mwait in C1 will
> >    allow the other thread to run faster.
> 
> Sounds plausible.

Plausible, but wrong. Tests showed that CPU use
during the peak load of this test increased from
about 71% to about 78% with this change, or just
under 10% increase relative to the baseline.

Coincidentally, that is the same CPU use increase
I have seen with the poll_idle() changes. Not sure
if that means anything...

> > 2) Insert more cpu_relax() calls into the
> >    main loop, so the CPU core spends more
> >    of its time in cpu_relax() and less
> >    time doing other things:
> 
> Well, maybe it's a matter of doing cpu_relax() between any other bits
> of
> significant computation in there:

I tried that, as well, and some other variations.

Every single change to poll_idle() that I tried
seems to result in a 9-10% relative increase in
CPU use during the peak load of the test.

During the busiest parts of the load, every CPU
sees on the order of 20k context switches a second.

kind regards,

Rik
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