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

Johannes Weiner
2023-04-18T19:15:08Z

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;
+ 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;

--
2.39.2