Re: [PATCH] rcuperf: Don't treat gp_exp mis-setting as a WARN

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 25, 2016 at 09:25:33AM +0800, Boqun Feng wrote:
> 0day found a boot warning triggered in rcu_perf_writer() on !SMP kernel:
> 
> 	WARN_ON(rcu_gp_is_normal() && gp_exp);
> 
> , the root cause of which is trying to measure expedited grace
> periods(by setting gp_exp to true by default) when all the grace periods
> are normal(TINY RCU only has normal grace periods).
> 
> However, such a mis-setting would only result in failing to measure the
> performance for a specific kind of grace periods, therefore using a
> WARN_ON to check this is a little overkilling. We could handle this
> inside rcuperf module via some error messages to tell users about the
> mis-settings.
> 
> Therefore this patch removes the WARN_ON in rcu_perf_writer() and
> handles those checkings in rcu_perf_init() with plain if() code.
> 
> Moreover, this patch changes the default value of gp_exp to 1) align
> with rcutorture tests and 2) make the default setting work for all RCU
> implementations by default.
> 
> Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Fixes: http://lkml.kernel.org/r/57411b10.mFvG0+AgcrMXGtcj%fengguang.wu@intel.com

Queued for review and testing, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/rcuperf.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> index 3cee0d8393ed..8ce4eecff319 100644
> --- a/kernel/rcu/rcuperf.c
> +++ b/kernel/rcu/rcuperf.c
> @@ -58,7 +58,7 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.vnet.ibm.com>");
>  #define VERBOSE_PERFOUT_ERRSTRING(s) \
>  	do { if (verbose) pr_alert("%s" PERF_FLAG "!!! %s\n", perf_type, s); } while (0)
> 
> -torture_param(bool, gp_exp, true, "Use expedited GP wait primitives");
> +torture_param(bool, gp_exp, false, "Use expedited GP wait primitives");
>  torture_param(int, holdoff, 10, "Holdoff time before test start (s)");
>  torture_param(int, nreaders, -1, "Number of RCU reader threads");
>  torture_param(int, nwriters, -1, "Number of RCU updater threads");
> @@ -363,8 +363,6 @@ rcu_perf_writer(void *arg)
>  	u64 *wdpp = writer_durations[me];
> 
>  	VERBOSE_PERFOUT_STRING("rcu_perf_writer task started");
> -	WARN_ON(rcu_gp_is_expedited() && !rcu_gp_is_normal() && !gp_exp);
> -	WARN_ON(rcu_gp_is_normal() && gp_exp);
>  	WARN_ON(!wdpp);
>  	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
>  	sp.sched_priority = 1;
> @@ -631,6 +629,16 @@ rcu_perf_init(void)
>  		firsterr = -ENOMEM;
>  		goto unwind;
>  	}
> +	if (rcu_gp_is_expedited() && !rcu_gp_is_normal() && !gp_exp) {
> +		VERBOSE_PERFOUT_ERRSTRING("All grace periods expedited, no normal ones to measure!");
> +		firsterr = -EINVAL;
> +		goto unwind;
> +	}
> +	if (rcu_gp_is_normal() && gp_exp) {
> +		VERBOSE_PERFOUT_ERRSTRING("All grace periods normal, no expedited ones to measure!");
> +		firsterr = -EINVAL;
> +		goto unwind;
> +	}
>  	for (i = 0; i < nrealwriters; i++) {
>  		writer_durations[i] =
>  			kcalloc(MAX_MEAS, sizeof(*writer_durations[i]),
> -- 
> 2.8.2
>