linux-kernel - Re: [PATCH] sched/loadavg: Avoid loadavg spikes caused by delayed NO

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Message-ID: <20170215161619.GD28416@codeblueprint.co.uk>
Date:   Wed, 15 Feb 2017 16:16:19 +0000
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        stable@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/loadavg: Avoid loadavg spikes caused by delayed
 NO_HZ accounting

On Wed, 15 Feb, at 04:12:11PM, Frederic Weisbecker wrote:
> On Wed, Feb 08, 2017 at 01:29:24PM +0000, Matt Fleming wrote:
> > The calculation for the next sample window when exiting NOH_HZ idle
> > does not handle the fact that we may not have reached the next sample
> > window yet
> 
> That sentence is hard to parse, it took me some time to figure out that
> those two "next sample window" may not refer to the same thing.
 
Yeah, it's not the most lucid thing I've ever written.

> Maybe it would be clearer with something along the lines of:
> 
> "The calculation for the next sample window when exiting NO_HZ
>  does not handle the fact that we may not have crossed any sample
>  window during the NO_HZ period."
 
Umm... this isn't the problem. In fact, it's the opposite.

The problem is that if we *did* cross a sample window while in NO_HZ,
then when we exit the pending window may be far enough into the future
that all we need to do is update this_rq->calc_load_update.

> > If we wake from NO_HZ idle after the pending this_rq->calc_load_update
> > window time when we want idle but before the next sample window
> 
> That too was hard to understand. How about:
> 
> "If we enter in NO_HZ mode after a pending this_rq->calc_load_update
>  and we exit from NO_HZ mode before the forthcoming sample window, ..."
 
You've got this backwards again. We enter NO_HZ before the pending
window, not after.