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@xxxxxxxxxx> AuthorDate: Mon, 25 May 2020 09:22:24 -07:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxxxx> --- 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@xxxxxxxxxxxxx>"); * 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)
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