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Date:   Thu, 11 Apr 2019 08:42:39 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 0/4] Allow CPU0 to be nohz full

On Tue, Apr 09, 2019 at 07:21:54PM +1000, Nicholas Piggin wrote:
> Thomas Gleixner's on April 6, 2019 3:54 am:
> > 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.
> 
> Okay.
> 
> > 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.
> 
> I will try to make the changelogs less jibberish then :)

Maybe this is all taken care of now, but do the various clocks stay
synchronized with wall-clock time if all CPUs are in nohz_full mode?
At one time, at least one CPU needed to keep its scheduler-clock
interrupt going in order to keep things in sync.

The ppc timebase register might make it possible to do this without any
scheduler-clock interrupts, but figured I should check.  ;-)

							Thanx, Paul

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