linux-kernel - Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS

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Message-ID: <20171222091221.ow5vn3ydx3hj4nht@hirez.programming.kicks-ass.net>
Date:   Fri, 22 Dec 2017 10:12:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On Fri, Dec 22, 2017 at 09:29:15AM +0100, Peter Zijlstra wrote:
> On Fri, Dec 22, 2017 at 09:05:45AM +0100, Vincent Guittot wrote:
> > On 22 December 2017 at 08:59, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Thu, Dec 21, 2017 at 05:56:32PM +0100, Vincent Guittot wrote:
> > >> In fact, we can't only rely on the tick and newly_idle load balance to
> > >> ensure a period update of the blocked load because they can never
> > >> happen.
> > >
> > > I'm confused, why would the ilb not happen?
> > 
> > the ilb will be kick only if tick fires which might not be the case
> > for task that runs less than a tick
> 
> Oh, urgh, you're talking about when the entire system is idle. Yes
> indeed.
> 
> Lemme have a think, surely we can do something saner there.

The only thing I could come up with is running a timer for this :/ That
would keep the ILB thing running until all load is decayed (have a patch
for that somewhere).