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. P. > > _______________________________________________ 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
