Re: [PATCH] mm: huge_memory: don't start_stop_khugepaged for non-PMD THP

[Barry Song <21cnbao@gmail.com>]

On Fri, Jul 26, 2024 at 9:48 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 26.07.24 11:43, Ryan Roberts wrote:
> > On 26/07/2024 09:28, Barry Song wrote:
> >> From: Barry Song <v-songbaohua@oppo.com>
> >>
> >> khugepaged will be automatically started when PMD-sized THP is enabled
> >> (either of the per-size anon control or the top-level control are set
> >> to "always" or "madvise"), and it'll be automatically shutdown when
> >> PMD-sized THP is disabled (when both the per-size anon control and the
> >> top-level control are "never").
> >>
> >> It seems unnecessary to call start_stop_khugepaged() for non-PMD THP,
> >> as it would only waste CPU time.
> >>
> >> Cc: Lance Yang <ioworker0@gmail.com>
> >> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> >> Cc: David Hildenbrand <david@redhat.com>
> >> Cc: Yang Shi <shy828301@gmail.com>
> >> Cc: Zi Yan <ziy@nvidia.com>
> >> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> >> ---
> >>   mm/huge_memory.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 41460847988c..bd365e35acf7 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -514,7 +514,7 @@ static ssize_t thpsize_enabled_store(struct kobject *kobj,
> >>      } else
> >>              ret = -EINVAL;
> >>
> >> -    if (ret > 0) {
> >> +    if (ret > 0 && order == HPAGE_PMD_ORDER) {
> >>              int err;
> >>
> >>              err = start_stop_khugepaged();
> >
> > Personally I see this as a bit of a layering violation; its
> > start_stop_khugepaged() that should decide the policy for when to start and stop
> > the daemon. thpsize_enabled_store() should just be calling
> > start_stop_khugepaged() to notify that something potentially pertinent to the a
> > policy decision has changed.
>

My impression is that it slightly deviates from the huge page documentation in
Documentation/admin-guide/mm/transhuge.rst.

khugepaged will be automatically started when PMD-sized THP is enabled
(either of the per-size anon control or the top-level control are set
to "always" or "madvise"), and it'll be automatically shutdown when
PMD-sized THP is disabled (when both the per-size anon control and the
top-level control are "never").

non-PMD size is not involved in khugepaged, but I agree the policy might change
in the future.

> Agreed, skimming the subject I was under the impression that we would be
> fixing something here.

working on another swapin_enabled and reviewing the enabled source code.
I don't need this startstop for all sizes in that case, so I made a
quick adjustment
to this part as well. If neither of you likes it, that's fine with me :-)

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry