linux-kernel - Re: [PATCH] rcuperf: Make rcuperf kernel test more robust for !expedited mode

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Date:   Thu, 4 Jul 2019 10:40:44 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH] rcuperf: Make rcuperf kernel test more robust for
 !expedited mode

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
> 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@...lfernandes.org>

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
>