[PATCH] zsmalloc: allow only one active pool compaction context

[Sergey Senozhatsky]

zsmalloc pool can be compacted concurrently by many contexts,
e.g.

 cc1 handle_mm_fault()
      do_anonymous_page()
       __alloc_pages_slowpath()
        try_to_free_pages()
         do_try_to_free_pages(
          lru_gen_shrink_node()
           shrink_slab()
            do_shrink_slab()
             zs_shrinker_scan()
              zs_compact()

This creates unnecessary contention as all those processes
compete for access to the same classes. A single compaction
process is enough. Moreover contention that is created by
multiple compaction processes impact other zsmalloc functions,
e.g. zs_malloc(), since zsmalloc uses "global" pool->lock to
synchronize access to pool.

Introduce pool compaction mutex and permit only one compaction
context at a time.

/proc/lock-stat after make -j$((`nproc`+1)) linux kernel for
&pool->lock#3:

                Base           Patched
--------------------------------------
con-bounces     9797655        8125860
contentions     11131185       9242153
waittime-min    0.09           0.10
waittime-max    4171695.76     3926258.74
waittime-total  506197629.16   417061026.20
waittime-avg    45.48          45.13
acq-bounces     13809103       11383480
acquisitions    21145155       18049364
holdtime-min    0.06           0.07
holdtime-max    7379928.80     3926274.89
holdtime-total  46273950.89    37279624.53
holdtime-avg    2.19           2.07

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 mm/zsmalloc.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index cc81dfba05a0..0e036ec56c3c 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -264,6 +264,7 @@ struct zs_pool {
 	struct work_struct free_work;
 #endif
 	spinlock_t lock;
+	struct mutex compact_lock;
 };
 
 struct zspage {
@@ -2274,6 +2275,9 @@ unsigned long zs_compact(struct zs_pool *pool)
 	struct size_class *class;
 	unsigned long pages_freed = 0;
 
+	if (!mutex_trylock(&pool->compact_lock))
+		return 0;
+
 	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
 		class = pool->size_class[i];
 		if (class->index != i)
@@ -2281,6 +2285,7 @@ unsigned long zs_compact(struct zs_pool *pool)
 		pages_freed += __zs_compact(pool, class);
 	}
 	atomic_long_add(pages_freed, &pool->stats.pages_compacted);
+	mutex_unlock(&pool->compact_lock);
 
 	return pages_freed;
 }
@@ -2388,6 +2393,7 @@ struct zs_pool *zs_create_pool(const char *name)
 
 	init_deferred_free(pool);
 	spin_lock_init(&pool->lock);
+	mutex_init(&pool->compact_lock);
 
 	pool->name = kstrdup(name, GFP_KERNEL);
 	if (!pool->name)
-- 
2.40.0.634.g4ca3ef3211-goog