Re: [PATCH 1/2] rcutorture: Disable RT throttling for boost tests

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

On Sun, Jun 10, 2018 at 04:45:43PM -0700, Joel Fernandes wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> 
> Currently rcutorture is not able to torture RCU boosting properly. This
> is because the rcutorture's boost threads which are doing the torturing
> may be throttled due to RT throttling.
> 
> This patch makes rcutorture use the right torture technique (unthrottled
> rcutorture boost tasks) for torturing RCU so that the test fails
> correctly when no boost is available.
> 
> Currently this requires accessing sysctl_sched_rt_runtime directly, but
> that should be Ok since rcutorture is test code. Such direct access is
> also only possible if rcutorture is used as a built-in so make it
> conditional on that.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you, Joel!  I have queued these for testing and review.
Peter Zijlstra might have some feedback on variable access, but will
test first.

							Thanx, Paul

> ---
>  kernel/rcu/rcutorture.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 5219e7ccd8a4..00e06349d7bb 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -55,6 +55,7 @@
>  #include <linux/torture.h>
>  #include <linux/vmalloc.h>
>  #include <linux/sched/debug.h>
> +#include <linux/sched/sysctl.h>
> 
>  #include "rcu.h"
> 
> @@ -772,6 +773,32 @@ static void rcu_torture_boost_cb(struct rcu_head *head)
>  	smp_store_release(&rbip->inflight, 0);
>  }
> 
> +static int old_rt_runtime = -1;
> +
> +static void rcu_torture_disable_rt_throttle(void)
> +{
> +	/*
> +	 * Disable RT throttling so that rcutorture's boost threads don't get
> +	 * throttled. Only possible if rcutorture is built-in otherwise the
> +	 * user should manually do this by setting the sched_rt_period_us and
> +	 * sched_rt_runtime sysctls.
> +	 */
> +	if (!IS_BUILTIN(CONFIG_RCU_TORTURE_TEST) || old_rt_runtime != -1)
> +		return;
> +
> +	old_rt_runtime = sysctl_sched_rt_runtime;
> +	sysctl_sched_rt_runtime = -1;
> +}
> +
> +static void rcu_torture_enable_rt_throttle(void)
> +{
> +	if (!IS_BUILTIN(CONFIG_RCU_TORTURE_TEST) || old_rt_runtime == -1)
> +		return;
> +
> +	sysctl_sched_rt_runtime = old_rt_runtime;
> +	old_rt_runtime = -1;
> +}
> +
>  static int rcu_torture_boost(void *arg)
>  {
>  	unsigned long call_rcu_time;
> @@ -1510,6 +1537,7 @@ static int rcutorture_booster_cleanup(unsigned int cpu)
>  	mutex_lock(&boost_mutex);
>  	t = boost_tasks[cpu];
>  	boost_tasks[cpu] = NULL;
> +	rcu_torture_enable_rt_throttle();
>  	mutex_unlock(&boost_mutex);
> 
>  	/* This must be outside of the mutex, otherwise deadlock! */
> @@ -1526,6 +1554,7 @@ static int rcutorture_booster_init(unsigned int cpu)
> 
>  	/* Don't allow time recalculation while creating a new task. */
>  	mutex_lock(&boost_mutex);
> +	rcu_torture_disable_rt_throttle();
>  	VERBOSE_TOROUT_STRING("Creating rcu_torture_boost task");
>  	boost_tasks[cpu] = kthread_create_on_node(rcu_torture_boost, NULL,
>  						  cpu_to_node(cpu),
> -- 
> 2.18.0.rc1.242.g61856ae69a-goog
>