linux-kernel - Re: [PATCH 0/4] Allow CPU0 to be nohz full

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Message-Id: <1554800737.v126tflazd.astroid@bobo.none>
Date:   Tue, 09 Apr 2019 19:21:54 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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

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 :)

Thanks,
Nick