linux-kernel - Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant from nohz

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Message-ID: <20170815130712.GA16627@lerouge>
Date:   Tue, 15 Aug 2017 15:07:14 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant
 from nohz

On Mon, Aug 14, 2017 at 08:29:46PM +0200, Mike Galbraith wrote:
> On Mon, 2017-08-14 at 13:34 -0400, Luiz Capitulino wrote:
> > On Mon, 14 Aug 2017 19:01:09 +0200
> > 
> > > What is the source of the load balancing inducing such latency when a single
> > > task is affine to a CPU? If this is idle load balancing, it is now affine to
> > > housekeepers. If this is task wakeup then it's suprising because select_task_rq()
> > > is optimized toward single CPU affinity.
> > 
> > I guess it was idle load balancing, but I don't remember because this
> > was a few years ago. I think this might be reproducible without using
> > isolcpus=. I'll give it a try shortly and let you know.
> 
> idle_balance() can swamp other noise by a couple orders of magnitude,

Ah I missed that one. Is there any way we can also lower the overhead there?
It looks unfortunately hard to tell if there is only one task affine to a given CPU,
assertion on top of which we could make a fast exit.