zsmalloc: allow only one active pool compaction context

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()

Pool compaction is currently (basically) single-threaded as
it is performed under pool->lock. Having multiple compaction
threads results in unnecessary contention, as each thread
competes for pool->lock. This, in turn, affects all zsmalloc
operations such as zs_malloc(), zs_map_object(), zs_free(), etc.

Introduce the pool->compaction_in_progress atomic variable,
which ensures that only one compaction context can run at a
time. This reduces overall pool->lock contention in (corner)
cases when many contexts attempt to shrink zspool simultaneously.

Link: https://lkml.kernel.org/r/20230418074639.1903197-1-senozhatsky@chromium.org
Fixes: c0547d0 ("zsmalloc: consolidate zs_pool's migrate_lock and size_class's locks")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sergey-senozhatsky

mm/zsmalloc.c

@@ -264,6 +264,7 @@ struct zs_pool {
 	struct work_struct free_work;
 #endif
 	spinlock_t lock;
+	atomic_t compaction_in_progress;
 };
 
 struct zspage {
@@ -2274,13 +2275,23 @@ unsigned long zs_compact(struct zs_pool *pool)
 	struct size_class *class;
 	unsigned long pages_freed = 0;
 
+	/*
+	 * Pool compaction is performed under pool->lock so it is basically
+	 * single-threaded. Having more than one thread in __zs_compact()
+	 * will increase pool->lock contention, which will impact other
+	 * zsmalloc operations that need pool->lock.
+	 */
+	if (atomic_xchg(&pool->compaction_in_progress, 1))
+		return 0;
+
 	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
 		class = pool->size_class[i];
 		if (class->index != i)
 			continue;
 		pages_freed += __zs_compact(pool, class);
 	}
 	atomic_long_add(pages_freed, &pool->stats.pages_compacted);
+	atomic_set(&pool->compaction_in_progress, 0);
 
 	return pages_freed;
 }
@@ -2388,6 +2399,7 @@ struct zs_pool *zs_create_pool(const char *name)
 
 	init_deferred_free(pool);
 	spin_lock_init(&pool->lock);
+	atomic_set(&pool->compaction_in_progress, 0);
 
 	pool->name = kstrdup(name, GFP_KERNEL);
 	if (!pool->name)