Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events properly

[David Rientjes]

On Mon, 1 Feb 2021, Charan Teja Reddy wrote:

> By defination, COMPACT[STALL|FAIL] events needs to be counted when there

s/defination/definition/

> is 'At least in one zone compaction wasn't deferred or skipped from the
> direct compaction'. And when compaction is skipped or deferred,
> COMPACT_SKIPPED will be returned but it will still go and update these
> compaction events which is wrong in the sense that COMPACT[STALL|FAIL]
> is counted without even trying the compaction.
> 
> Correct this by skipping the counting of these events when
> COMPACT_SKIPPED is returned for compaction. This indirectly also avoid
> the unnecessary try into the get_page_from_freelist() when compaction is
> not even tried.
> 
> Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
> ---
>  mm/page_alloc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 519a60d..531f244 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4152,6 +4152,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  	memalloc_noreclaim_restore(noreclaim_flag);
>  	psi_memstall_leave(&pflags);
>  
> +	if (*compact_result == COMPACT_SKIPPED)
> +		return NULL;
>  	/*
>  	 * At least in one zone compaction wasn't deferred or skipped, so let's
>  	 * count a compaction stall

This makes sense, I wonder if it would also be useful to check that 
page == NULL, either in try_to_compact_pages() or here for 
COMPACT_SKIPPED?