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>

1 files changed, 47 insertions, 12 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7c8cc5141877..c447c29f8749 100644
--- a/kernel/cgroup.c
+++ b/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;