Messages in this thread | | | Date | Fri, 5 Apr 2019 19:54:30 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH 0/4] Allow CPU0 to be nohz full |
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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
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