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.
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