[PATCH tip/core/rcu 03/30] rcuperf: Add comments explaining the high reader overhead

paulmck
2020-06-23T00:32:22Z

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@kernel.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@linux.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)
--
2.9.5