It is possible that the rcuperf kernel test runs concurrently with init starting up. During this time, the system is running all grace periods as expedited. However, rcuperf can also be run for normal GP tests. Right now, it depends on a holdoff time before starting the test to ensure grace periods start later. This works fine with the default holdoff time however it is not robust in situations where init takes greater than the holdoff time to finish running. Or, as in my case: I modified the rcuperf test locally to also run a thread that did preempt disable/enable in a loop. This had the effect of slowing down init. The end result was that the "batches:" counter in rcuperf was 0 causing a division by 0 error in the results. This counter was 0 because only expedited GPs seem to happen, not normal ones which led to the rcu_state.gp_seq counter remaining constant across grace periods which unexpectedly happen to be expedited. The system was running expedited RCU all the time because rcu_unexpedited_gp() would not have run yet from init. In other words, the test would concurrently with init booting in expedited GP mode. To fix this properly, let us check if system_state if SYSTEM_RUNNING is set before starting the test. The system_state approximately aligns with when rcu_unexpedited_gp() is called and works well in practice. I also tried late_initcall however it is still too early to be meaningful for this case. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- kernel/rcu/rcuperf.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c index 4513807cd4c4..5a879d073c1c 100644 --- a/kernel/rcu/rcuperf.c +++ b/kernel/rcu/rcuperf.c @@ -375,6 +375,14 @@ rcu_perf_writer(void *arg) if (holdoff) schedule_timeout_uninterruptible(holdoff * HZ); + /* + * Wait until rcu_end_inkernel_boot() is called for normal GP tests + * so that RCU is not always expedited for normal GP tests. + * The system_state test is approximate, but works well in practice. + */ + while (!gp_exp && system_state != SYSTEM_RUNNING) + schedule_timeout_uninterruptible(1); + t = ktime_get_mono_fast_ns(); if (atomic_inc_return(&n_rcu_perf_writer_started) >= nrealwriters) { t_rcu_perf_writer_started = t; -- 2.22.0.410.gd8fdbe21b5-goog