[PATCH] mm, slub: splice cpu and page freelists in deactivate_slab()

[PATCH] mm, slub: splice cpu and page freelists in deactivate_slab()

[Vlastimil Babka]

In deactivate_slab() we currently move all but one objects on the cpu freelist
to the page freelist one by one using the costly cmpxchg_double() operation.
Then we unfreeze the page while moving the last object on page freelist, with
a final cmpxchg_double().

This can be optimized to avoid the cmpxchg_double() per object. Just count the
objects on cpu freelist (to adjust page->inuse properly) and also remember the
last object in the chain. Then splice page->freelist to the last object and
effectively add the whole cpu freelist to page->freelist while unfreezing the
page, with a single cmpxchg_double().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Hi,

I stumbled on the optimization while pondering over what to do with the percpu
partial list memory wastage [1], but it should be useful on its own. I haven't
run any measurements yet, but eliminating cmpxchg_double() operations should be
obviously faster [TM]. Passed some basic testing, including hardened freelist
and slub_debug.

[1] https://lore.kernel.org/linux-mm/CAG48ez2Qx5K1Cab-m8BdSibp6wLTip6ro4=-umR7BLsEgjEYzA@mail.gmail.com/

 mm/slub.c | 59 ++++++++++++++++++++++---------------------------------
 1 file changed, 24 insertions(+), 35 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0d4bdf6783ee..c3141aa962be 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2167,9 +2167,9 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
 {
 	enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
 	struct kmem_cache_node *n = get_node(s, page_to_nid(page));
-	int lock = 0;
+	int lock = 0, free_delta = 0;
 	enum slab_modes l = M_NONE, m = M_NONE;
-	void *nextfree;
+	void *nextfree, *freelist_iter, *freelist_tail;
 	int tail = DEACTIVATE_TO_HEAD;
 	struct page new;
 	struct page old;
@@ -2180,45 +2180,34 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
 	}
 
 	/*
-	 * Stage one: Free all available per cpu objects back
-	 * to the page freelist while it is still frozen. Leave the
-	 * last one.
-	 *
-	 * There is no need to take the list->lock because the page
-	 * is still frozen.
+	 * Stage one: Count the objects on cpu's freelist as free_delta and
+	 * remember the last object in freelist_tail for later splicing.
 	 */
-	while (freelist && (nextfree = get_freepointer(s, freelist))) {
-		void *prior;
-		unsigned long counters;
+	freelist_tail = NULL;
+	freelist_iter = freelist;
+	while (freelist_iter) {
+		nextfree = get_freepointer(s, freelist_iter);
 
 		/*
 		 * If 'nextfree' is invalid, it is possible that the object at
-		 * 'freelist' is already corrupted.  So isolate all objects
-		 * starting at 'freelist'.
+		 * 'freelist_iter' is already corrupted.  So isolate all objects
+		 * starting at 'freelist_iter' by skipping them.
 		 */
-		if (freelist_corrupted(s, page, &freelist, nextfree))
+		if (freelist_corrupted(s, page, &freelist_iter, nextfree))
 			break;
 
-		do {
-			prior = page->freelist;
-			counters = page->counters;
-			set_freepointer(s, freelist, prior);
-			new.counters = counters;
-			new.inuse--;
-			VM_BUG_ON(!new.frozen);
+		freelist_tail = freelist_iter;
+		free_delta++;
 
-		} while (!__cmpxchg_double_slab(s, page,
-			prior, counters,
-			freelist, new.counters,
-			"drain percpu freelist"));
-
-		freelist = nextfree;
+		freelist_iter = nextfree;
 	}
 
 	/*
-	 * Stage two: Ensure that the page is unfrozen while the
-	 * list presence reflects the actual number of objects
-	 * during unfreeze.
+	 * Stage two: Unfreeze the page while splicing the per-cpu
+	 * freelist to the head of page's freelist.
+	 *
+	 * Ensure that the page is unfrozen while the list presence
+	 * reflects the actual number of objects during unfreeze. 
 	 *
 	 * We setup the list membership and then perform a cmpxchg
 	 * with the count. If there is a mismatch then the page
@@ -2231,15 +2220,15 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
 	 */
 redo:
 
-	old.freelist = page->freelist;
-	old.counters = page->counters;
+	old.freelist = READ_ONCE(page->freelist);
+	old.counters = READ_ONCE(page->counters);
 	VM_BUG_ON(!old.frozen);
 
 	/* Determine target state of the slab */
 	new.counters = old.counters;
-	if (freelist) {
-		new.inuse--;
-		set_freepointer(s, freelist, old.freelist);
+	if (freelist_tail) {
+		new.inuse -= free_delta;
+		set_freepointer(s, freelist_tail, old.freelist);
 		new.freelist = freelist;
 	} else
 		new.freelist = old.freelist;
-- 
2.29.2

Re: [PATCH] mm, slub: splice cpu and page freelists in deactivate_slab()

[Jann Horn]

On Fri, Jan 15, 2021 at 7:35 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> In deactivate_slab() we currently move all but one objects on the cpu freelist
> to the page freelist one by one using the costly cmpxchg_double() operation.
> Then we unfreeze the page while moving the last object on page freelist, with
> a final cmpxchg_double().
>
> This can be optimized to avoid the cmpxchg_double() per object. Just count the
> objects on cpu freelist (to adjust page->inuse properly) and also remember the
> last object in the chain. Then splice page->freelist to the last object and
> effectively add the whole cpu freelist to page->freelist while unfreezing the
> page, with a single cmpxchg_double().

This might have some more (good) effects, although these might well be
too minuscule to notice:

 - The old code inverted the direction of the freelist, while the new
code preserves the direction.
 - We're no longer dirtying the cachelines of objects in the middle of
the freelist.

In the current code it probably doesn't really matter, since I think
we basically only take this path when handling NUMA mismatches,
PFMEMALLOC stuff, racing new_slab(), and flush_slab() for handling
flushing IPIs? But yeah, if we want to start automatically sending
flush IPIs, it might be a good idea, given that the next accesses to
the page will probably come from a different CPU (unless the page is
entirely unused, in which case it may be freed to the page allocator's
percpu list) and we don't want to create unnecessary cache/memory
traffic. (And it's a good cleanup regardless, I think.)

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Reviewed-by: Jann Horn <jannh@google.com>

[...]
>         /*
> -        * Stage two: Ensure that the page is unfrozen while the
> -        * list presence reflects the actual number of objects
> -        * during unfreeze.
> +        * Stage two: Unfreeze the page while splicing the per-cpu
> +        * freelist to the head of page's freelist.
> +        *
> +        * Ensure that the page is unfrozen while the list presence
> +        * reflects the actual number of objects during unfreeze.

(my computer complains about trailing whitespace here)

>          *
>          * We setup the list membership and then perform a cmpxchg
>          * with the count. If there is a mismatch then the page