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

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Date:   Mon, 26 Mar 2018 17:48:22 -0400
From:   Rik van Riel <riel@...riel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 Mon, 2018-03-26 at 23:44 +0200, Rafael J. Wysocki wrote:
> On Mon, Mar 26, 2018 at 6:32 PM, Rik van Riel <riel@...riel.com>
> wrote:
> > On Sun, 2018-03-25 at 23:34 +0200, Rafael J. Wysocki wrote:
> > 
> > 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.

... and I did something wrong in the tests :/

> Hmm.  Basically, you are saying that
> 
> while (something)
>     cpu_relax();
> 
> is measurably less overhead than
> 
> while (something) {
>     cpu_relax();
>     check something else;
>     cpu_relax();
> }
> 
> which honestly makes me wonder how this is possible at all.

Part of the mystery is solved. Apparently the
control kernel was not the one I thought it
was, or the one that was printed in the 
output messages :(

I am re-running the tests now with a few
variations, and seeing what works best.

My apologies for wasting everybody's time.

I should have valid results later tonight.
Fingers crossed.

-- 
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