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

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/07/2024 11:04, Barry Song wrote:
> 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").

But start_stop_khugepaged() doesn't unconditionally start khugepaged, it takes
action based on hugepage_pmd_enabled() which only returns true if there are any
pmd sized THP enabled (currently looking at anon and file, but should also look
at shmem in future; that's a known bug that's been there forever). So I don't
think it is inconsistent with the documentation?

> 
> 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