Re: [RFC PATCH 14/26] mm: compaction: simplify should_compact_retry()

["Huang, Ying" <ying.huang@intel.com>]

Johannes Weiner <hannes@cmpxchg.org> writes:

> The different branches for retry are unnecessarily complicated. There
> is really only three outcomes: progress, skipped, failed. Also, the
> retry counter only applies to loops that made progress, move it there.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/page_alloc.c | 60 +++++++++++++++++--------------------------------
>  1 file changed, 20 insertions(+), 40 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c3b7dc479936..18fa2bbba44b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4608,7 +4608,6 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  		     enum compact_priority *compact_priority,
>  		     int *compaction_retries)
>  {
> -	int max_retries = MAX_COMPACT_RETRIES;
>  	int min_priority;
>  	bool ret = false;
>  	int retries = *compaction_retries;
> @@ -4621,19 +4620,27 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  		return false;
>  
>  	/*
> -	 * Compaction managed to coalesce some page blocks, but the
> -	 * allocation failed presumably due to a race. Retry some.
> +	 * Compaction coalesced some page blocks, but the allocation
> +	 * failed, presumably due to a race. Retry a few times.
>  	 */
> -	if (compact_result == COMPACT_SUCCESS)
> -		(*compaction_retries)++;
> +	if (compact_result == COMPACT_SUCCESS) {
> +		int max_retries = MAX_COMPACT_RETRIES;
>  
> -	/*
> -	 * All zones were scanned completely and still no result. It
> -	 * doesn't really make much sense to retry except when the
> -	 * failure could be caused by insufficient priority
> -	 */
> -	if (compact_result == COMPACT_COMPLETE)
> -		goto check_priority;
> +		/*
> +		 * !costly requests are much more important than
> +		 * __GFP_RETRY_MAYFAIL costly ones because they are de
> +		 * facto nofail and invoke OOM killer to move on while
> +		 * costly can fail and users are ready to cope with
> +		 * that. 1/4 retries is rather arbitrary but we would
> +		 * need much more detailed feedback from compaction to
> +		 * make a better decision.
> +		 */
> +		if (order > PAGE_ALLOC_COSTLY_ORDER)
> +			max_retries /= 4;
> +
> +		ret = ++(*compaction_retries) <= MAX_COMPACT_RETRIES;
                                                 ~~~~~~~~~~~~~~~~~~~

Should be max_retries?

Best Regards,
Huang, Ying

> +		goto out;
> +	}
>  
>  	/*
>  	 * Compaction was skipped due to a lack of free order-0
> @@ -4645,35 +4652,8 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	}
>  
>  	/*
> -	 * If compaction backed due to being deferred, due to
> -	 * contended locks in async mode, or due to scanners meeting
> -	 * after a partial scan, retry with increased priority.
> -	 */
> -	if (compact_result == COMPACT_DEFERRED ||
> -	    compact_result == COMPACT_CONTENDED ||
> -	    compact_result == COMPACT_PARTIAL_SKIPPED)
> -		goto check_priority;
> -
> -	/*
> -	 * !costly requests are much more important than __GFP_RETRY_MAYFAIL
> -	 * costly ones because they are de facto nofail and invoke OOM
> -	 * killer to move on while costly can fail and users are ready
> -	 * to cope with that. 1/4 retries is rather arbitrary but we
> -	 * would need much more detailed feedback from compaction to
> -	 * make a better decision.
> -	 */
> -	if (order > PAGE_ALLOC_COSTLY_ORDER)
> -		max_retries /= 4;
> -	if (*compaction_retries <= max_retries) {
> -		ret = true;
> -		goto out;
> -	}
> -
> -	/*
> -	 * Make sure there are attempts at the highest priority if we exhausted
> -	 * all retries or failed at the lower priorities.
> +	 * Compaction failed. Retry with increasing priority.
>  	 */
> -check_priority:
>  	min_priority = (order > PAGE_ALLOC_COSTLY_ORDER) ?
>  			MIN_COMPACT_COSTLY_PRIORITY : MIN_COMPACT_PRIORITY;