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: c0547d0b6a4b ("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>

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index cc81dfb..44ddaf5 100644 (file)
--- a/mm/zsmalloc.c
+++ b/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 {
@@ -2274,6 +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)