From: Thomas Gleixner <tglx@linutronix.de>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>
Subject: Re: [PATCH 0/4] Allow CPU0 to be nohz full
Date: Fri, 5 Apr 2019 19:54:30 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.21.1904051952160.1802@nanos.tec.linutronix.de> (raw)
In-Reply-To: <1554393113.wbjxx9ccdx.astroid@bobo.none>
On Fri, 5 Apr 2019, Nicholas Piggin wrote:
> Thomas Gleixner's on April 5, 2019 12:36 am:
> > On Thu, 4 Apr 2019, Nicholas Piggin wrote:
> >
> >> I've been looking at ways to fix suspend breakage with CPU0 as a
> >> nohz CPU. I started looking at various things like allowing CPU0
> >> to take over do_timer again temporarily or allowing nohz full
> >> to be stopped at runtime (that is quite a significant change for
> >> little real benefit). The problem then was having the housekeeping
> >> CPU go offline.
> >>
> >> So I decided to try just allowing the freeze to occur on non-zero
> >> CPU. This seems to be a lot simpler to get working, but I guess
> >> some archs won't be able to deal with this? Would it be okay to
> >> make it opt-in per arch?
> >
> > It needs to be opt in. x86 will fall on its nose with that.
>
> Okay I can add that.
>
> > Now the real interesting question is WHY do we need that at all?
>
> Why full nohz for CPU0? Basically this is how their job system was
> written and used, testing nohz full was a change that came much later
> as an optimisation.
>
> I don't think there is a fundamental reason an equivalent system
> could not be made that uses a different CPU for housekeeping, but I
> was assured the change would be quite difficult for them.
>
> If we can support it, it seems nice if you can take a particular
> configuration and just apply nohz_full to your application processors
> without any other changes.
This wants an explanation in the patches. And patch 4 has in the changelog:
nohz_full has been successful at significantly reducing jitter for a
large supercomputer customer, but their job control system requires CPU0
to be for housekeeping.
which just makes me dazed and confused :)
Other than some coherent explanation and making it opt in, I don't think
there is a fundamental issue with that.
Thanks,
tglx
next prev parent reply other threads:[~2019-04-05 17:54 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-04 12:07 [PATCH 0/4] Allow CPU0 to be nohz full Nicholas Piggin
2019-04-04 12:07 ` [PATCH 1/4] sched/core: allow the remote scheduler tick to be started on CPU0 Nicholas Piggin
2019-04-04 12:07 ` [PATCH 2/4] kernel/cpu: Allow non-zero CPU to be primary for suspend / kexec freeze Nicholas Piggin
2019-04-04 12:07 ` [PATCH 3/4] kernel/sched/isolation: require a present CPU in housekeeping mask Nicholas Piggin
2019-04-04 12:07 ` [PATCH 4/4] nohz_full: Allow the boot CPU to be full nohz Nicholas Piggin
2019-04-04 14:36 ` [PATCH 0/4] Allow CPU0 to be nohz full Thomas Gleixner
2019-04-04 16:02 ` Nicholas Piggin
2019-04-05 17:54 ` Thomas Gleixner [this message]
2019-04-09 9:21 ` Nicholas Piggin
2019-04-11 15:42 ` Paul E. McKenney
2019-04-12 3:16 ` Nicholas Piggin
2019-04-12 11:30 ` Paul E. McKenney
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