linux-kernel - Re: [PATCH] tick/nohz: Narrow down noise while setting current task's tick dependency

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Message-ID: <20200517162001.GA30646@lenoir>
Date:   Sun, 17 May 2020 18:20:02 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Fleming <matt@...eblueprint.co.uk>, stable@...nel.org
Subject: Re: [PATCH] tick/nohz: Narrow down noise while setting current
 task's tick dependency

On Sun, May 17, 2020 at 08:53:22AM -0700, Paul E. McKenney wrote:
> On Sun, May 17, 2020 at 03:31:16PM +0200, Frederic Weisbecker wrote:
> > On Fri, May 15, 2020 at 08:07:18PM -0700, Paul E. McKenney wrote:
> > > On Fri, May 15, 2020 at 02:34:29AM +0200, Frederic Weisbecker wrote:
> > > > So far setting a tick dependency on any task, including current, used to
> > > > trigger an IPI to all CPUs. That's of course suboptimal but it wasn't
> > > > an issue as long as it was only used by posix-cpu-timers on nohz_full,
> > > > a combo that nobody seemed to use in real life.
> > > > 
> > > > But RCU started to use task tick dependency on current task to fix
> > > > stall issues on callbacks processing. These trigger regular and
> > > > undesired system wide IPIs on nohz_full.
> > > > 
> > > > The fix is very easy while setting a tick dependency on the current
> > > > task, only its CPU needs an IPI.
> > > 
> > > This passes moderate rcutorture testing.  If you want me to take it, please
> > > let me know, and otherwise:
> > > 
> > > Tested-by: Paul E. McKenney <paulmck@...nel.org>
> > 
> > If you already have a pending urgent queue, I'd love you to take it.
> > If not I can take it.
> 
> Nothing urgent yet in -rcu, so if you would like it in the next merge
> window, please take it through your normal upstream path.

Got it, thanks!