> On 12/12/19 9:10 AM, Justin Forbes wrote:
> > On Thu, Dec 12, 2019 at 3:48 AM Peter Robinson <pbrobinson(a)gmail.com>
wrote:
> >>
> >> 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.
This wasn't a proposal to change anything here at all, sorry if that
was the way it read. I was purely wondering, while digging through
stuff around cpu idle, for the difference between arches.
With the hit around NO_HZ_IDLE vs NO_HZ_FULL I dug some more and
basically it seems the reason we don't have the later on the non
x86_64 arches is because for some reason we unset
VIRT_CPU_ACCOUNTING_GEN for all except x86_64, it looks to be
historical, all our current architectures now look to support that
option. Anyone aware of any reason we shouldn't use the
NO_HZ_FULL/VIRT_CPU_ACCOUNTING_GEN as standard across all arches?
P