cgroup: Avoid false cacheline sharing of read mostly rstat_cpu

The rstat_cpu and also rstat_css_list of the cgroup structure are read
mostly variables. However, they may share the same cacheline as the
subsequent rstat_flush_next and *bstat variables which can be updated
frequently.  That will slow down the cgroup_rstat_cpu() call which is
called pretty frequently in the rstat code. Add a CACHELINE_PADDING()
line in between them to avoid false cacheline sharing.

A parallel kernel build on a 2-socket x86-64 server is used as the
benchmarking tool for measuring the lock hold time. Below were the lock
hold time frequency distribution before and after the patch:

      Run time        Before patch       After patch
      --------        ------------       -----------
       0-01 us         9,928,562          9,820,428
      01-05 us           110,151             50,935
      05-10 us               270                 93
      10-15 us               273                146
      15-20 us               135                 76
      20-25 us                 0                  2
      25-30 us                 1                  0

It can be seen that the patch further pushes the lock hold time towards
the lower end.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Waiman-Long

include/linux/cgroup-defs.h

@@ -496,6 +496,13 @@ struct cgroup {
 	struct cgroup_rstat_cpu __percpu *rstat_cpu;
 	struct list_head rstat_css_list;
 
+	/*
+	 * Add padding to separate the read mostly rstat_cpu and
+	 * rstat_css_list into a different cacheline from the following
+	 * rstat_flush_next and *bstat fields which can have frequent updates.
+	 */
+	CACHELINE_PADDING(_pad_);
+
 	/*
 	 * A singly-linked list of cgroup structures to be rstat flushed.
 	 * This is a scratch field to be used exclusively by