linux-kernel - [RFC PATCH v4 7/9] slub: Optimize deactivate

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Date:   Tue, 31 Oct 2023 14:07:39 +0000
From:   chengming.zhou@...ux.dev
To:     vbabka@...e.cz, cl@...ux.com, penberg@...nel.org,
        willy@...radead.org
Cc:     rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, chengming.zhou@...ux.dev,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [RFC PATCH v4 7/9] slub: Optimize deactivate_slab()

From: Chengming Zhou <zhouchengming@...edance.com>

Since the introduce of unfrozen slabs on cpu partial list, we don't
need to synchronize the slab frozen state under the node list_lock.

The caller of deactivate_slab() and the caller of __slab_free() won't
manipulate the slab list concurrently.

So we can get node list_lock in the last stage if we really need to
manipulate the slab list in this path.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 mm/slub.c | 76 +++++++++++++++++++------------------------------------
 1 file changed, 26 insertions(+), 50 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index bcb5b2c4e213..c429f8baba5f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2468,10 +2468,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
 static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
 			    void *freelist)
 {
-	enum slab_modes { M_NONE, M_PARTIAL, M_FREE, M_FULL_NOLIST };
 	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
 	int free_delta = 0;
-	enum slab_modes mode = M_NONE;
 	void *nextfree, *freelist_iter, *freelist_tail;
 	int tail = DEACTIVATE_TO_HEAD;
 	unsigned long flags = 0;
@@ -2512,62 +2510,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
 	 *
 	 * Ensure that the slab is unfrozen while the list presence
 	 * reflects the actual number of objects during unfreeze.
-	 *
-	 * We first perform cmpxchg holding lock and insert to list
-	 * when it succeed. If there is mismatch then the slab is not
-	 * unfrozen and number of objects in the slab may have changed.
-	 * Then release lock and retry cmpxchg again.
 	 */
-redo:
-
-	old.freelist = READ_ONCE(slab->freelist);
-	old.counters = READ_ONCE(slab->counters);
-	VM_BUG_ON(!old.frozen);
-
-	/* Determine target state of the slab */
-	new.counters = old.counters;
-	if (freelist_tail) {
-		new.inuse -= free_delta;
-		set_freepointer(s, freelist_tail, old.freelist);
-		new.freelist = freelist;
-	} else
-		new.freelist = old.freelist;
-
-	new.frozen = 0;
+	do {
+		old.freelist = READ_ONCE(slab->freelist);
+		old.counters = READ_ONCE(slab->counters);
+		VM_BUG_ON(!old.frozen);
+
+		/* Determine target state of the slab */
+		new.counters = old.counters;
+		new.frozen = 0;
+		if (freelist_tail) {
+			new.inuse -= free_delta;
+			set_freepointer(s, freelist_tail, old.freelist);
+			new.freelist = freelist;
+		} else {
+			new.freelist = old.freelist;
+		}
+	} while (!slab_update_freelist(s, slab,
+		old.freelist, old.counters,
+		new.freelist, new.counters,
+		"unfreezing slab"));
 
+	/*
+	 * Stage three: Manipulate the slab list based on the updated state.
+	 */
 	if (!new.inuse && n->nr_partial >= s->min_partial) {
-		mode = M_FREE;
+		stat(s, DEACTIVATE_EMPTY);
+		discard_slab(s, slab);
+		stat(s, FREE_SLAB);
 	} else if (new.freelist) {
-		mode = M_PARTIAL;
-		/*
-		 * Taking the spinlock removes the possibility that
-		 * acquire_slab() will see a slab that is frozen
-		 */
 		spin_lock_irqsave(&n->list_lock, flags);
-	} else {
-		mode = M_FULL_NOLIST;
-	}
-
-
-	if (!slab_update_freelist(s, slab,
-				old.freelist, old.counters,
-				new.freelist, new.counters,
-				"unfreezing slab")) {
-		if (mode == M_PARTIAL)
-			spin_unlock_irqrestore(&n->list_lock, flags);
-		goto redo;
-	}
-
-
-	if (mode == M_PARTIAL) {
 		add_partial(n, slab, tail);
 		spin_unlock_irqrestore(&n->list_lock, flags);
 		stat(s, tail);
-	} else if (mode == M_FREE) {
-		stat(s, DEACTIVATE_EMPTY);
-		discard_slab(s, slab);
-		stat(s, FREE_SLAB);
-	} else if (mode == M_FULL_NOLIST) {
+	} else {
 		stat(s, DEACTIVATE_FULL);
 	}
 }
-- 
2.20.1