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, 6 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2d1d151258cf..f585b7cde87b 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -156,6 +156,12 @@ 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];
-
 };
 
 /*