cgroups: use a hash table for css_set finding

When we attach a process to a different cgroup, the css_set linked-list will
be run through to find a suitable existing css_set to use.  This patch
implements a hash table for better performance.

The following benchmarks have been tested:

For N in 1, 5, 10, 50, 100, 500, 1000, create N cgroups with one sleeping
task in each, and then move an additional task through each cgroup in
turn.

Here is a test result:

N	Loop	orig - Time(s)	hash - Time(s)
----------------------------------------------
1	10000	1.201231728	1.196311177
5	2000	1.065743872	1.040566424
10	1000	0.991054735	0.986876440
50	200	0.976554203	0.969608733
100	100	0.998504680	0.969218270
500	20	1.157347764	0.962602963
1000	10	1.619521852	1.085140172

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cgroup.h

@@ -155,6 +155,12 @@ struct css_set {
 	 */
 	struct list_head list;
 
+	/*
+	 * List running through all cgroup groups in the same hash
+	 * slot. Protected by css_set_lock
+	 */
+	struct hlist_node hlist;
+
 	/*
 	 * List running through all tasks using this cgroup
 	 * group. Protected by css_set_lock
@@ -174,7 +180,6 @@ struct css_set {
 	 * during subsystem registration (at boot time).
 	 */
 	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
-
 };
 
 /*

kernel/cgroup.c

@@ -44,6 +44,7 @@
 #include <linux/kmod.h>
 #include <linux/delayacct.h>
 #include <linux/cgroupstats.h>
+#include <linux/hash.h>
 
 #include <asm/atomic.h>
 
@@ -193,6 +194,27 @@ static struct cg_cgroup_link init_css_set_link;
 static DEFINE_RWLOCK(css_set_lock);
 static int css_set_count;
 
+/* hash table for cgroup groups. This improves the performance to
+ * find an existing css_set */
+#define CSS_SET_HASH_BITS	7
+#define CSS_SET_TABLE_SIZE	(1 << CSS_SET_HASH_BITS)
+static struct hlist_head css_set_table[CSS_SET_TABLE_SIZE];
+
+static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
+{
+	int i;
+	int index;
+	unsigned long tmp = 0UL;
+
+	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
+		tmp += (unsigned long)css[i];
+	tmp = (tmp >> 16) ^ tmp;
+
+	index = hash_long(tmp, CSS_SET_HASH_BITS);
+
+	return &css_set_table[index];
+}
+
 /* We don't maintain the lists running through each css_set to its
  * task until after the first call to cgroup_iter_start(). This
  * reduces the fork()/exit() overhead for people who have cgroups
@@ -219,6 +241,7 @@ static int use_task_css_set_links;
 static void unlink_css_set(struct css_set *cg)
 {
 	write_lock(&css_set_lock);
+	hlist_del(&cg->hlist);
 	list_del(&cg->list);
 	css_set_count--;
 	while (!list_empty(&cg->cg_links)) {
@@ -284,9 +307,7 @@ static inline void put_css_set_taskexit(struct css_set *cg)
 /*
  * find_existing_css_set() is a helper for
  * find_css_set(), and checks to see whether an existing
- * css_set is suitable. This currently walks a linked-list for
- * simplicity; a later patch will use a hash table for better
- * performance
+ * css_set is suitable.
  *
  * oldcg: the cgroup group that we're using before the cgroup
  * transition
@@ -303,7 +324,9 @@ static struct css_set *find_existing_css_set(
 {
 	int i;
 	struct cgroupfs_root *root = cgrp->root;
-	struct list_head *l = &init_css_set.list;
+	struct hlist_head *hhead;
+	struct hlist_node *node;
+	struct css_set *cg;
 
 	/* Built the set of subsystem state objects that we want to
 	 * see in the new css_set */
@@ -320,18 +343,13 @@ static struct css_set *find_existing_css_set(
 		}
 	}
 
-	/* Look through existing cgroup groups to find one to reuse */
-	do {
-		struct css_set *cg =
-			list_entry(l, struct css_set, list);
-
+	hhead = css_set_hash(template);
+	hlist_for_each_entry(cg, node, hhead, hlist) {
 		if (!memcmp(template, cg->subsys, sizeof(cg->subsys))) {
 			/* All subsystems matched */
 			return cg;
 		}
-		/* Try the next cgroup group */
-		l = l->next;
-	} while (l != &init_css_set.list);
+	}
 
 	/* No existing cgroup group matched */
 	return NULL;
@@ -393,6 +411,8 @@ static struct css_set *find_css_set(
 	struct list_head tmp_cg_links;
 	struct cg_cgroup_link *link;
 
+	struct hlist_head *hhead;
+
 	/* First see if we already have a cgroup group that matches
 	 * the desired set */
 	write_lock(&css_set_lock);
@@ -417,6 +437,7 @@ static struct css_set *find_css_set(
 	kref_init(&res->ref);
 	INIT_LIST_HEAD(&res->cg_links);
 	INIT_LIST_HEAD(&res->tasks);
+	INIT_HLIST_NODE(&res->hlist);
 
 	/* Copy the set of subsystem state objects generated in
 	 * find_existing_css_set() */
@@ -459,6 +480,11 @@ static struct css_set *find_css_set(
 	/* Link this cgroup group into the list */
 	list_add(&res->list, &init_css_set.list);
 	css_set_count++;
+
+	/* Add this cgroup group to the hash table */
+	hhead = css_set_hash(res->subsys);
+	hlist_add_head(&res->hlist, hhead);
+
 	write_unlock(&css_set_lock);
 
 	return res;
@@ -2508,6 +2534,7 @@ int __init cgroup_init_early(void)
 	INIT_LIST_HEAD(&init_css_set.list);
 	INIT_LIST_HEAD(&init_css_set.cg_links);
 	INIT_LIST_HEAD(&init_css_set.tasks);
+	INIT_HLIST_NODE(&init_css_set.hlist);
 	css_set_count = 1;
 	init_cgroup_root(&rootnode);
 	list_add(&rootnode.root_list, &roots);
@@ -2520,6 +2547,9 @@ int __init cgroup_init_early(void)
 	list_add(&init_css_set_link.cg_link_list,
 		 &init_css_set.cg_links);
 
+	for (i = 0; i < CSS_SET_TABLE_SIZE; i++)
+		INIT_HLIST_HEAD(&css_set_table[i]);
+
 	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
 		struct cgroup_subsys *ss = subsys[i];
 
@@ -2549,6 +2579,7 @@ int __init cgroup_init(void)
 {
 	int err;
 	int i;
+	struct hlist_head *hhead;
 
 	err = bdi_init(&cgroup_backing_dev_info);
 	if (err)
@@ -2560,6 +2591,10 @@ int __init cgroup_init(void)
 			cgroup_init_subsys(ss);
 	}
 
+	/* Add init_css_set to the hash table */
+	hhead = css_set_hash(init_css_set.subsys);
+	hlist_add_head(&init_css_set.hlist, hhead);
+
 	err = register_filesystem(&cgroup_fs_type);
 	if (err < 0)
 		goto out;