On Mon, Dec 16, 2019 at 8:33 AM Prarit Bhargava <prarit@xxxxxxxxxx> wrote: > > On 12/13/19 11:35 AM, Justin Forbes wrote: > > On Fri, Dec 13, 2019 at 8:14 AM Laura Abbott <labbott@xxxxxxxxxx> wrote: > >> > >> On 12/12/19 9:10 AM, Justin Forbes wrote: > >>> On Thu, Dec 12, 2019 at 3:48 AM Peter Robinson <pbrobinson@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> > >>> 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. Justin _______________________________________________ kernel mailing list -- kernel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to kernel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/kernel@xxxxxxxxxxxxxxxxxxxxxxx
