linux-kernel - Re: [PATCH v3 3/3] sched: update blocked load when newly idle

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Message-ID: <20180301160918.GC29639@lerouge>
Date:   Thu, 1 Mar 2018 17:09:20 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, valentin.schneider@....com,
        morten.rasmussen@...s.arm.com, brendan.jackman@....com,
        dietmar.eggemann@....com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v3 3/3] sched: update blocked load when newly idle

On Mon, Feb 12, 2018 at 04:38:05PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 12, 2018 at 03:34:44PM +0100, Vincent Guittot wrote:
> > > Aside from the above being an unreadable mess, I dislike that it breaks
> > > the various isolation crud, we should not touch CPUs outside of our
> > > domain.
> > >
> > > 
> > > Maybe something like the below? (unfinished)
> > >
> > 
> > good catch. I completely miss the isolation stuff.
> > But isn't already the case when kicking ilb ? I mean that an idle CPU touches
> > all idle CPUs and some can be outside its domain during ilb.
> 
> > Shouldn't we test housekeeping_cpu(cpu, HK_FLAG_SCHED) instead if we want to
> > make sure that an isolated/full nohz CPU will not be used for updating blocked
> > load of CPUs outside its domain ?
> 
> I _thought_ we had some 'housekeeping' crud in the ilb selection logic,
> but now I can't find it. Frederic?

I think you're referring to nohz_balance_idle(). The call is still there but HK_FLAG_SCHED
is unused for now. I initially turned it on by default on nohz_full but some people
complained. I don't recall why exactly. Anyway I'm waiting for a suitable interface
to use it.