linux-kernel - Re: [RFC PATCH 12/12] housekeeping: Reimplement isolcpus on housekeeping

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Date:   Fri, 1 Sep 2017 01:00:00 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Rik van Riel <riel@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [RFC PATCH 12/12] housekeeping: Reimplement isolcpus on
 housekeeping

On Thu, Aug 31, 2017 at 08:53:56PM +0200, Thomas Gleixner wrote:
> On Mon, 28 Aug 2017, Frederic Weisbecker wrote:
> > On Mon, Aug 28, 2017 at 06:24:16PM +0200, Peter Zijlstra wrote:
> > > On Mon, Aug 28, 2017 at 05:27:15PM +0200, Frederic Weisbecker wrote:
> > > > Although for example I guess (IIUC) that if you create an unbound
> > > > timer on a NULL domain, it will be stuck on it for ever as we can't
> > > > walk any hierarchy from the current CPU domain.
> > > 
> > > Not sure what you're on about. Timers have their own hierarchy.
> > 
> > Check out get_nohz_timer_target() which relies on scheduler hierarchies to
> > look up a CPU to enqueue an unpinned timer on.
> 
> Which is one of the most idiotic things we have in that code
> path. Anna-Maria has posted this series which gets rid of that nonsense, by
> queueing the timer on the current cpu into a wheel, which gets pulled in by
> others. That makes a lot of sense because most of these timers get canceled
> before expiry anyway. But we still need to fix the fallout and the few
> corner cases to make that work reliably. We'll do that hopefully sooner
> than later.

Sure, I definetly agree with that change.