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authorDomenico Cerasuolo <cerasuolodomenico@gmail.com>2023-05-26 20:32:27 +0200
committerAndrew Morton <akpm@linux-foundation.org>2023-06-09 16:25:51 -0700
commite0228d590beb0d0af345c58a282f01afac5c57f3 (patch)
tree45373dbf1aa65a59f27d119d2e389b18ead4bcd4 /mm/zswap.c
parente3d9b45fb17cfddb1c414b5981743d4245fcf486 (diff)
mm: zswap: shrink until can accept
This update addresses an issue with the zswap reclaim mechanism, which hinders the efficient offloading of cold pages to disk, thereby compromising the preservation of the LRU order and consequently diminishing, if not inverting, its performance benefits. The functioning of the zswap shrink worker was found to be inadequate, as shown by basic benchmark test. For the test, a kernel build was utilized as a reference, with its memory confined to 1G via a cgroup and a 5G swap file provided. The results are presented below, these are averages of three runs without the use of zswap: real 46m26s user 35m4s sys 7m37s With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G system), the results changed to: real 56m4s user 35m13s sys 8m43s written_back_pages: 18 reject_reclaim_fail: 0 pool_limit_hit:1478 Besides the evident regression, one thing to notice from this data is the extremely low number of written_back_pages and pool_limit_hit. The pool_limit_hit counter, which is increased in zswap_frontswap_store when zswap is completely full, doesn't account for a particular scenario: once zswap hits his limit, zswap_pool_reached_full is set to true; with this flag on, zswap_frontswap_store rejects pages if zswap is still above the acceptance threshold. Once we include the rejections due to zswap_pool_reached_full && !zswap_can_accept(), the number goes from 1478 to a significant 21578266. Zswap is stuck in an undesirable state where it rejects pages because it's above the acceptance threshold, yet fails to attempt memory reclaimation. This happens because the shrink work is only queued when zswap_frontswap_store detects that it's full and the work itself only reclaims one page per run. This state results in hot pages getting written directly to disk, while cold ones remain memory, waiting only to be invalidated. The LRU order is completely broken and zswap ends up being just an overhead without providing any benefits. This commit applies 2 changes: a) the shrink worker is set to reclaim pages until the acceptance threshold is met and b) the task is also enqueued when zswap is not full but still above the threshold. Testing this suggested update showed much better numbers: real 36m37s user 35m8s sys 9m32s written_back_pages: 10459423 reject_reclaim_fail: 12896 pool_limit_hit: 75653 Link: https://lkml.kernel.org/r/20230526183227.793977-1-cerasuolodomenico@gmail.com Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit") Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Yosry Ahmed <yosryahmed@google.com> Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com> Cc: Dan Streetman <ddstreet@ieee.org> Cc: Seth Jennings <sjenning@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm/zswap.c')
-rw-r--r--mm/zswap.c17
1 files changed, 14 insertions, 3 deletions
diff --git a/mm/zswap.c b/mm/zswap.c
index 59da2a415fbb..bcb82e09eb64 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -37,6 +37,7 @@
#include <linux/workqueue.h>
#include "swap.h"
+#include "internal.h"
/*********************************
* statistics
@@ -587,9 +588,19 @@ static void shrink_worker(struct work_struct *w)
{
struct zswap_pool *pool = container_of(w, typeof(*pool),
shrink_work);
+ int ret, failures = 0;
- if (zpool_shrink(pool->zpool, 1, NULL))
- zswap_reject_reclaim_fail++;
+ do {
+ ret = zpool_shrink(pool->zpool, 1, NULL);
+ if (ret) {
+ zswap_reject_reclaim_fail++;
+ if (ret != -EAGAIN)
+ break;
+ if (++failures == MAX_RECLAIM_RETRIES)
+ break;
+ }
+ cond_resched();
+ } while (!zswap_can_accept());
zswap_pool_put(pool);
}
@@ -1188,7 +1199,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
if (zswap_pool_reached_full) {
if (!zswap_can_accept()) {
ret = -ENOMEM;
- goto reject;
+ goto shrink;
} else
zswap_pool_reached_full = false;
}