On Thu, Jul 04, 2019 at 10:40:44AM -0700, Paul E. McKenney wrote: > On Thu, Jul 04, 2019 at 12:34:30AM -0400, Joel Fernandes (Google) wrote: > > 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 Just minor typo.. To fix this properly, let us check if system_state if SYSTEM_RUNNING is set before starting the test. ... Should be To fix this properly, let us check if system_state is set to SYSTEM_RUNNING before starting the test. ... Thanks, Byungchul > > 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> > > Good catch, queued, thank you! > > Thanx, Paul > > > --- > > 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 > >