linux-kernel - [tip: core/rcu] rcuperf: Add comments explaining the high reader overhead

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Message-ID: <159618738930.4006.13348759385369896937.tip-bot2@tip-bot2>
Date:   Fri, 31 Jul 2020 09:23:09 -0000
From:   "tip-bot2 for Paul E. McKenney" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] rcuperf: Add comments explaining the high reader overhead

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     708cda31652c02e64adaeafafe7b996e4e14c3eb
Gitweb:        https://git.kernel.org/tip/708cda31652c02e64adaeafafe7b996e4e14c3eb
Author:        Paul E. McKenney <paulmck@...nel.org>
AuthorDate:    Mon, 25 May 2020 09:22:24 -07:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 29 Jun 2020 12:00:44 -07:00

rcuperf: Add comments explaining the high reader overhead

This commit adds comments explaining why the readers have otherwise insane
levels of measurement overhead, namely that they are intended as a test
load for update-side performance measurements, not as a straight-up
read-side performance test.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/rcuperf.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index 246da8f..d906ca9 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -69,6 +69,11 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@...ux.ibm.com>");
  *	value specified by nr_cpus for a read-only test.
  *
  * Various other use cases may of course be specified.
+ *
+ * Note that this test's readers are intended only as a test load for
+ * the writers.  The reader performance statistics will be overly
+ * pessimistic due to the per-critical-section interrupt disabling,
+ * test-end checks, and the pair of calls through pointers.
  */
 
 #ifdef MODULE
@@ -309,8 +314,10 @@ static void rcu_perf_wait_shutdown(void)
 }
 
 /*
- * RCU perf reader kthread.  Repeatedly does empty RCU read-side
- * critical section, minimizing update-side interference.
+ * RCU perf reader kthread.  Repeatedly does empty RCU read-side critical
+ * section, minimizing update-side interference.  However, the point of
+ * this test is not to evaluate reader performance, but instead to serve
+ * as a test load for update-side performance testing.
  */
 static int
 rcu_perf_reader(void *arg)