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

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Message-ID: <2958394.SolTkngEXc@aspire.rjw.lan>
Date:   Thu, 22 Mar 2018 18:24:28 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Rik van Riel <riel@...riel.com>
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 Thursday, March 22, 2018 6:19:16 PM CET Rik van Riel wrote:
> On Thu, 2018-03-22 at 18:09 +0100, Rafael J. Wysocki wrote:
> > On Thursday, March 22, 2018 5:32:23 PM CET Rik van Riel wrote:
> > > On Wed, 2018-03-14 at 15:08 +0100, Rafael J. Wysocki wrote:
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > 
> > > > If poll_idle() is allowed to spin until need_resched() returns
> > > > 'true',
> > > > it may actually spin for a much longer time than expected by the
> > > > idle
> > > > governor, since set_tsk_need_resched() is not always called by
> > > > the
> > > > timer interrupt handler.  If that happens, the CPU may spend much
> > > > more time than anticipated in the "polling" state.
> > > > 
> > > > To prevent that from happening, limit the time of the spinning
> > > > loop
> > > > in poll_idle().
> > > > 
> > > > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> > > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > 
> > > So ... about bisecting that other patch series...
> > > 
> > > It turned out I had this patch, which looks so
> > > obviously correct, as patch #1 in my series.
> > > 
> > > It also turned out that this patch is responsible
> > > for the entire 5-10% increase in CPU use for the
> > > memcache style workload.
> > > 
> > > I wonder if keeping an idle HT thread much busier
> > > than before slows down its sibling, or something
> > > like that.
> > 
> > Uhm, sorry about this.
> 
> No worries, this is why we do patch reviews and
> tests in the first place.
> 
> > Does it improve if you do something like the below on top of it?
> 
> That was my next thing to try, after testing just
> the idle nohz series by itself :)
> 
> I'll push both into the test systems, and will
> get back to you when I have answers.

Thanks!