From: Paul E. McKenney Date: Mon, 25 May 2020 16:22:24 +0000 (-0700) Subject: rcuperf: Add comments explaining the high reader overhead X-Git-Tag: block-5.9-2020-08-14~74^2^2^5~27 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=708cda31652c02e64adaeafafe7b996e4e14c3eb;p=linux-block.git 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 --- diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c index 246da8fe199e..d906ca987936 100644 --- a/kernel/rcu/rcuperf.c +++ b/kernel/rcu/rcuperf.c @@ -69,6 +69,11 @@ MODULE_AUTHOR("Paul E. McKenney "); * 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)