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

Huang, Ying
2023-04-25T00:58:00Z

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;