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From:   Waiman Long <longman@redhat.com>
To:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Cc:     cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Joe Mario <jmario@redhat.com>,
        Sebastian Jug <sejug@redhat.com>,
        Yosry Ahmed <yosryahmed@google.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH v3 3/3] cgroup: Avoid false cacheline sharing of read mostly rstat_cpu
Date:   Fri,  3 Nov 2023 23:13:03 -0400
Message-Id: <20231104031303.592879-4-longman@redhat.com>
In-Reply-To: <20231104031303.592879-1-longman@redhat.com>
References: <20231104031303.592879-1-longman@redhat.com>
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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        14,594,545         15,484,707
      01-05 us           439,926            207,382
      05-10 us             5,960              3,174
      10-15 us             3,543              3,006
      15-20 us             1,397              1,066
      20-25 us                25                 15
      25-30 us                12                 10

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>
---
 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 ff4b4c590f32..a4adc0580135 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -491,6 +491,13 @@ 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