[v6 PATCH 11/11] mm: vmscan: shrink deferred objects proportional to priority

[Yang Shi]

The number of deferred objects might get windup to an absurd number, and it
results in clamp of slab objects.  It is undesirable for sustaining workingset.

So shrink deferred objects proportional to priority and cap nr_deferred to twice
of cache items.

The idea is borrowed fron Dave Chinner's patch:
https://lore.kernel.org/linux-xfs/20191031234618.15403-13-david@xxxxxxxxxxxxx/

Tested with kernel build and vfs metadata heavy workload in our production
environment, no regression is spotted so far.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/vmscan.c | 40 +++++-----------------------------------
 1 file changed, 5 insertions(+), 35 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 574d920c4cab..d0a86170854b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -649,7 +649,6 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	 */
 	nr = count_nr_deferred(shrinker, shrinkctl);
 
-	total_scan = nr;
 	if (shrinker->seeks) {
 		delta = freeable >> priority;
 		delta *= 4;
@@ -663,37 +662,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		delta = freeable / 2;
 	}
 
+	total_scan = nr >> priority;
 	total_scan += delta;
-	if (total_scan < 0) {
-		pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n",
-		       shrinker->scan_objects, total_scan);
-		total_scan = freeable;
-		next_deferred = nr;
-	} else
-		next_deferred = total_scan;
-
-	/*
-	 * We need to avoid excessive windup on filesystem shrinkers
-	 * due to large numbers of GFP_NOFS allocations causing the
-	 * shrinkers to return -1 all the time. This results in a large
-	 * nr being built up so when a shrink that can do some work
-	 * comes along it empties the entire cache due to nr >>>
-	 * freeable. This is bad for sustaining a working set in
-	 * memory.
-	 *
-	 * Hence only allow the shrinker to scan the entire cache when
-	 * a large delta change is calculated directly.
-	 */
-	if (delta < freeable / 4)
-		total_scan = min(total_scan, freeable / 2);
-
-	/*
-	 * Avoid risking looping forever due to too large nr value:
-	 * never try to free more than twice the estimate number of
-	 * freeable entries.
-	 */
-	if (total_scan > freeable * 2)
-		total_scan = freeable * 2;
+	total_scan = min(total_scan, (2 * freeable));
 
 	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
 				   freeable, delta, total_scan, priority);
@@ -732,10 +703,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		cond_resched();
 	}
 
-	if (next_deferred >= scanned)
-		next_deferred -= scanned;
-	else
-		next_deferred = 0;
+	next_deferred = max_t(long, (nr - scanned), 0) + total_scan;
+	next_deferred = min(next_deferred, (2 * freeable));
+
 	/*
 	 * move the unused scan count back into the shrinker in a
 	 * manner that handles concurrent updates.
-- 
2.26.2