Re: [PATCH v6 06/11] mm, compaction: more reliably increase direct compaction priority

[Joonsoo Kim]

On Wed, Aug 10, 2016 at 11:12:21AM +0200, Vlastimil Babka wrote:
> During reclaim/compaction loop, compaction priority can be increased by the
> should_compact_retry() function, but the current code is not optimal. Priority
> is only increased when compaction_failed() is true, which means that compaction
> has scanned the whole zone. This may not happen even after multiple attempts
> with a lower priority due to parallel activity, so we might needlessly
> struggle on the lower priorities and possibly run out of compaction retry
> attempts in the process.
> 
> After this patch we are guaranteed at least one attempt at the highest
> compaction priority even if we exhaust all retries at the lower priorities.

The only difference that this patch makes is increasing priority when
COMPACT_PARTIAL(COMPACTION_SUCCESS) returns. In that case, we can
usually allocate high-order freepage so we would not enter here. Am I
missing something? Is it really needed behaviour change?

Thanks.

> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/page_alloc.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fb975cec3518..b28517b918b0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3155,13 +3155,8 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	 * so it doesn't really make much sense to retry except when the
>  	 * failure could be caused by insufficient priority
>  	 */
> -	if (compaction_failed(compact_result)) {
> -		if (*compact_priority > MIN_COMPACT_PRIORITY) {
> -			(*compact_priority)--;
> -			return true;
> -		}
> -		return false;
> -	}
> +	if (compaction_failed(compact_result))
> +		goto check_priority;
>  
>  	/*
>  	 * make sure the compaction wasn't deferred or didn't bail out early
> @@ -3185,6 +3180,15 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	if (compaction_retries <= max_retries)
>  		return true;
>  
> +	/*
> +	 * Make sure there is at least one attempt at the highest priority
> +	 * if we exhausted all retries at the lower priorities
> +	 */
> +check_priority:
> +	if (*compact_priority > MIN_COMPACT_PRIORITY) {
> +		(*compact_priority)--;
> +		return true;
> +	}
>  	return false;

The only difference that this patch makes is increasing priority when
COMPACT_PARTIAL(COMPACTION_SUCCESS) returns. In that case, we can
usually allocate high-order freepage so we would not enter here. Am I
missing something? Is it really needed behaviour change?

Thanks.