From: Peter Zijlstra <peterz@infradead.org>
To: Daniel Vacek <neelx@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Valentin Schneider <vschneid@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair: properly serialize the cfs_rq h_load calculation
Date: Mon, 25 Nov 2024 11:01:08 +0100 [thread overview]
Message-ID: <20241125100108.GH24774@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <CAPjX3FfDTdUvMCDJCP8tAeNeaYSWj9mSsrMmE=VP0kWAdJTSVQ@mail.gmail.com>
On Fri, Nov 22, 2024 at 06:33:31PM +0100, Daniel Vacek wrote:
> On Fri, Nov 22, 2024 at 4:42 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Nov 22, 2024 at 04:28:55PM +0100, Daniel Vacek wrote:
> > > Make sure the given cfs_rq's h_load is always correctly updated. This
> > > prevents a race between more CPUs eventually updating the same hierarchy
> > > of h_load_next return pointers.
> >
> > Is there an actual problem observed?
>
> Well, that depends. Do we care about correct (exact) load calculation
> every time?
The whole load balancer is full of races. And typically it all more or
less works out.
I mean, the worst case is typically a spurious migration, which will get
'fixed' up the next round.
Only if behaviour gets to be really bad/stupid do we tend to try and fix
this.
Now your patch didn't look awful :-), but it would make a stronger case
if you'd done it because you observed it doing stupid and it now no
longer does stupid and your workload improves.
next prev parent reply other threads:[~2024-11-25 10:01 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-11-22 15:28 Daniel Vacek
2024-11-22 15:42 ` Peter Zijlstra
2024-11-22 17:33 ` Daniel Vacek
2024-11-25 10:01 ` Peter Zijlstra [this message]
2024-11-25 11:40 ` Daniel Vacek
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