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@xxxxxxxxxx>
---
include/linux/cgroup-defs.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 37518436cfe7..5a97ea95b564 100644
--- a/include/linux/cgroup-defs.h
+++ b/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
--
2.39.3
