Commit d2658f20 authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Andrew Morton
Browse files

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: c0547d0b ("zsmalloc: consolidate zs_pool's migrate_lock and size_class's locks")
Signed-off-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: default avatarYosry Ahmed <yosryahmed@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 07115fcc

mm/zsmalloc.c

+12 −0
Original line number Diff line number Diff line
@@ -264,6 +264,7 @@ struct zs_pool { 
	struct work_struct free_work;

#endif

	spinlock_t lock;

	atomic_t compaction_in_progress;

};



struct zspage {
@@ -2274,6 +2275,15 @@ 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)
@@ -2281,6 +2291,7 @@ unsigned long zs_compact(struct zs_pool *pool) 
		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)