> >>>> Hey All.
> >>>>
> >>>> In digging through some pieces around CPU_IDLE I noticed that
> >>>> NO_HZ_IDLE is explicitly disabled on x86_64 but not on all other
> >>>> architectures.
> >>>>
> >>>> Doing a "git log --follow
> >>>> configs/fedora/generic/x86/x86_64/CONFIG_NO_HZ_IDLE" it goes
all the
> >>>> way back to 2016 when we changed the way the configs were handled.
> >>>>
> >>>> The upstream kernel's opinion [1] on it is "Most of the
time you want
> >>>> to say Y here." so I'm wondering if there's a reason
why we're
> >>>> difference on x86_64 or is it just lost in the winds of time?
> >>>>
> >>>> Peter
> >>>>
> >>>> PS was digging around CPU_IDLE_GOV_TEO for those curious.
> >>>>
> >>>> [1]
https://cateee.net/lkddb/web-lkddb/NO_HZ_IDLE.html
> >>>
> >>>
> >>> commit 3836faf6e68495fc70316229a3540506f7ce4c98
> >>> Author: Kyle McMartin <kyle(a)fedoraproject.org>
> >>> Date: Wed Sep 17 13:10:12 2014 -0500
> >>>
> >>> re-enable RCU_FAST_NO_HZ, enable NO_HZ_FULL on x86_64
> >>>
> >>> - I also like to live dangerously. (Re-enable RCU_FAST_NO_HZ
which
> >>> has been off
> >>> since April 2012. Also enable NO_HZ_FULL on x86_64.)
> >>
> >> Yeah I wouldn't quite say it's been "lost" but the real
question
> >> is if it still makes sense. I don't have a strong opinion without
> >> data. Prarit, any opinion here?
> >
> > Oh, I wasn't pointing out that it wasn't just lost, I was pointing out
> > that NO_HZ_IDLE is not set because we run NO_HZ_FULL. We were one of
> > the first distros to do so, and it has worked well for us. I have a
> > fairly strong opinion about not dropping back to IDLE without good
> > reason.
>
> Getting back to the original question, I had to go back through my history to
> see if I could find a reason why there is a discrepancy between x86 and the
> other arches.
>
> AFAICT in *RHEL8* we have NO_HZ_FULL on all arches except s390x. S390x has
> NO_HZ_IDLE. Additionally s390 upstream has:
>
> [prarit@prarit linux]$ git grep NO_HZ_IDLE arch/s390/
> arch/s390/configs/debug_defconfig:4:CONFIG_NO_HZ_IDLE=y
> arch/s390/configs/defconfig:4:CONFIG_NO_HZ_IDLE=y
> arch/s390/configs/zfcpdump_defconfig:2:CONFIG_NO_HZ_IDLE=y
>
> On Fedora, as noted,
>
> [prarit@prarit fedora]$ find ./ -name *NO_HZ_IDLE* | xargs grep ^
> ./generic/x86/x86_64/CONFIG_NO_HZ_IDLE:# CONFIG_NO_HZ_IDLE is not set
> ./generic/CONFIG_NO_HZ_IDLE:CONFIG_NO_HZ_IDLE=y
> [prarit@prarit fedora]$ find ./ -name *NO_HZ_FULL* | xargs grep ^
> ./generic/x86/x86_64/CONFIG_NO_HZ_FULL:CONFIG_NO_HZ_FULL=y
> ./generic/CONFIG_NO_HZ_FULL:# CONFIG_NO_HZ_FULL is not set
>
> FWIW I think the correct thing to do for performance reasons is use NO_HZ_FULL
> on all arches except s390x which requires NO_HZ_IDLE.
>
Yes, I do believe this is the correct thing to do, as to how we got
into the current state, when NO_HZ_FULL was introduced, it was x86_64
only. Other architectures came in eventually, but as they were already
set to NO_HZ_IDLE, it didn't prompt us, and to be honest, we were
paying less attention to the other architectures back then. It has
been a while. I will get the changes made in rawhide with tomorrow's
builds.
It looks good on the arm architectures, thanks for everyone's feedback.
Peter