linux-kernel - Re: [PATCH -mm v2 1/3] slub: never fail to shrink cache

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Date:	Thu, 29 Jan 2015 21:21:41 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v2 1/3] slub: never fail to shrink cache

On Thu, Jan 29, 2015 at 10:22:16AM -0600, Christoph Lameter wrote:
> On Thu, 29 Jan 2015, Vladimir Davydov wrote:
> 
> > Yeah, but the tool just writes 1 to /sys/kernel/slab/cache/shrink, i.e.
> > invokes shrink_store(), and I don't propose to remove slab placement
> > optimization from there. What I propose is to move slab placement
> > optimization from kmem_cache_shrink() to shrink_store(), because other
> > users of kmem_cache_shrink() don't seem to need it at all - they just
> > want to release empty slabs. Such a change wouldn't affect the behavior
> > of `slabinfo -s` at all.
> 
> Well we have to go through the chain of partial slabs anyways so its easy
> to do the optimization at that point.

That's true, but we can introduce a separate function that would both
release empty slabs and optimize slab placement, like the patch below
does. It would increase the code size a bit though, so I don't insist.

diff --git a/mm/slub.c b/mm/slub.c
index 1562955fe099..2cd401d82a41 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3359,7 +3359,7 @@ void kfree(const void *x)
 EXPORT_SYMBOL(kfree);
 
 /*
- * kmem_cache_shrink removes empty slabs from the partial lists and sorts
+ * shrink_slab_cache removes empty slabs from the partial lists and sorts
  * the remaining slabs by the number of items in use. The slabs with the
  * most items in use come first. New allocations will then fill those up
  * and thus they can be removed from the partial lists.
@@ -3368,7 +3368,7 @@ EXPORT_SYMBOL(kfree);
  * being allocated from last increasing the chance that the last objects
  * are freed in them.
  */
-int __kmem_cache_shrink(struct kmem_cache *s)
+static int shrink_slab_cache(struct kmem_cache *s)
 {
 	int node;
 	int i;
@@ -3423,6 +3423,32 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 	return 0;
 }
 
+static int __kmem_cache_shrink(struct kmem_cache *s)
+{
+	int node;
+	struct kmem_cache_node *n;
+	struct page *page, *t;
+	LIST_HEAD(discard);
+	unsigned long flags;
+	int ret = 0;
+
+	flush_all(s);
+	for_each_kmem_cache_node(s, node, n) {
+		spin_lock_irqsave(&n->list_lock, flags);
+		list_for_each_entry_safe(page, t, &n->partial, lru)
+			if (!page->inuse)
+				list_move(&page->lru, &discard);
+		spin_unlock_irqrestore(&n->list_lock, flags);
+
+		list_for_each_entry_safe(page, t, &discard, lru)
+			discard_slab(s, page);
+
+		if (slabs_node(s, node))
+			ret = 1;
+	}
+	return ret;
+}
+
 static int slab_mem_going_offline_callback(void *arg)
 {
 	struct kmem_cache *s;
@@ -4683,7 +4709,7 @@ static ssize_t shrink_store(struct kmem_cache *s,
 			const char *buf, size_t length)
 {
 	if (buf[0] == '1') {
-		int rc = kmem_cache_shrink(s);
+		int rc = shrink_slab_cache(s);
 
 		if (rc)
 			return rc;
--
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