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

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

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.

And I don't think this should be a hot path; I'd expect the global thp size
configuration to be set at boot and remain pretty static. Do you have evidence
to the contrary?