linux-kernel - Re: [RT LATENCY] 249 microsecond latency caused by slub's unfreeze

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Date:	Thu, 28 Mar 2013 17:29:03 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clark@...hat.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [RT LATENCY] 249 microsecond latency caused by slub's
 unfreeze_partials() code.

Two patches against slub to enable deconfiguring cpu_partial support.

First one is a bug fix (Pekka please pick up this one or use Joonsoo's
earlier one).


Subject: slub: Fix object counts in acquire_slab

It seems that we were overallocating objects from the slab queues
since get_partial_node() assumed that page->inuse was undisturbed by
acquire_slab(). Save the # of objects in page->lru.next in acquire_slab()
and pass it to get_partial_node() that way.

I have a vague memory that Joonsoo also ran into this issue awhile back.

Signed-off-by: Christoph Lameter <cl@...ux.com>

Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c	2013-03-28 12:14:26.958358688 -0500
+++ linux/mm/slub.c	2013-03-28 12:16:57.240785613 -0500
@@ -1498,6 +1498,7 @@ static inline void *acquire_slab(struct
 	void *freelist;
 	unsigned long counters;
 	struct page new;
+	unsigned long objects;

 	/*
 	 * Zap the freelist and set the frozen bit.
@@ -1507,6 +1508,7 @@ static inline void *acquire_slab(struct
 	freelist = page->freelist;
 	counters = page->counters;
 	new.counters = counters;
+	objects = page->objects;
 	if (mode) {
 		new.inuse = page->objects;
 		new.freelist = NULL;
@@ -1524,6 +1526,7 @@ static inline void *acquire_slab(struct
 		return NULL;

 	remove_partial(n, page);
+	page->lru.next = (void *)objects;
 	WARN_ON(!freelist);
 	return freelist;
 }
@@ -1565,7 +1568,7 @@ static void *get_partial_node(struct kme
 			c->page = page;
 			stat(s, ALLOC_FROM_PARTIAL);
 			object = t;
-			available =  page->objects - page->inuse;
+			available =  page->objects - (unsigned long)page->lru.next;
 		} else {
 			available = put_cpu_partial(s, page, 0);
 			stat(s, CPU_PARTIAL_NODE);
--
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