linux-kernel - [PATCH 9/9] mm: remove unnecessary back-off function when retrying page reclaim

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Message-Id: <20170228214007.5621-10-hannes@cmpxchg.org>
Date:   Tue, 28 Feb 2017 16:40:07 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Jia He <hejianet@...il.com>, Michal Hocko <mhocko@...e.cz>,
        Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: [PATCH 9/9] mm: remove unnecessary back-off function when retrying page reclaim

The backoff mechanism is not needed. If we have MAX_RECLAIM_RETRIES
loops without progress, we'll OOM anyway; backing off might cut one or
two iterations off that in the rare OOM case. If we have intermittent
success reclaiming a few pages, the backoff function gets reset also,
and so is of little help in these scenarios.

We might want a backoff function for when there IS progress, but not
enough to be satisfactory. But this isn't that. Remove it.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/page_alloc.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9ac639864bed..223644afed28 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3511,11 +3511,10 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
 /*
  * Checks whether it makes sense to retry the reclaim to make a forward progress
  * for the given allocation request.
- * The reclaim feedback represented by did_some_progress (any progress during
- * the last reclaim round) and no_progress_loops (number of reclaim rounds without
- * any progress in a row) is considered as well as the reclaimable pages on the
- * applicable zone list (with a backoff mechanism which is a function of
- * no_progress_loops).
+ *
+ * We give up when we either have tried MAX_RECLAIM_RETRIES in a row
+ * without success, or when we couldn't even meet the watermark if we
+ * reclaimed all remaining pages on the LRU lists.
  *
  * Returns true if a retry is viable or false to enter the oom path.
  */
@@ -3560,13 +3559,11 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 		bool wmark;
 
 		available = reclaimable = zone_reclaimable_pages(zone);
-		available -= DIV_ROUND_UP((*no_progress_loops) * available,
-					  MAX_RECLAIM_RETRIES);
 		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
 
 		/*
-		 * Would the allocation succeed if we reclaimed the whole
-		 * available?
+		 * Would the allocation succeed if we reclaimed all
+		 * reclaimable pages?
 		 */
 		wmark = __zone_watermark_ok(zone, order, min_wmark,
 				ac_classzone_idx(ac), alloc_flags, available);
-- 
2.11.1