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Date:   Sat, 12 Nov 2022 16:54:53 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Connor O'Brien <connoro@google.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@android.com,
        John Stultz <jstultz@google.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [RFC PATCH 10/11] torture: support randomized shuffling for
 proxy exec testing
Message-ID: <Y2/P3cMExRt2fUP5@google.com>
References: <20221003214501.2050087-1-connoro@google.com>
 <20221003214501.2050087-11-connoro@google.com>
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On Mon, Oct 03, 2022 at 09:45:00PM +0000, Connor O'Brien wrote:
> Quick hack to better surface bugs in proxy execution.
> 
> Shuffling sets the same cpu affinities for all tasks, which makes us
> less likely to hit paths involving migrating blocked tasks onto a cpu
> where they can't run. Add an element of randomness to allow affinities
> of different writer tasks to diverge.
> 
> Signed-off-by: Connor O'Brien <connoro@google.com>
> ---
>  kernel/torture.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/torture.c b/kernel/torture.c
> index 789aeb0e1159..1d0dd88369e3 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -54,6 +54,9 @@ module_param(verbose_sleep_frequency, int, 0444);
>  static int verbose_sleep_duration = 1;
>  module_param(verbose_sleep_duration, int, 0444);
>  
> +static int random_shuffle;
> +module_param(random_shuffle, int, 0444);
> +
>  static char *torture_type;
>  static int verbose;
>  
> @@ -518,6 +521,7 @@ static void torture_shuffle_task_unregister_all(void)
>   */
>  static void torture_shuffle_tasks(void)
>  {
> +	DEFINE_TORTURE_RANDOM(rand);
>  	struct shuffle_task *stp;
>  
>  	cpumask_setall(shuffle_tmp_mask);
> @@ -537,8 +541,10 @@ static void torture_shuffle_tasks(void)
>  		cpumask_clear_cpu(shuffle_idle_cpu, shuffle_tmp_mask);
>  
>  	mutex_lock(&shuffle_task_mutex);
> -	list_for_each_entry(stp, &shuffle_task_list, st_l)
> -		set_cpus_allowed_ptr(stp->st_t, shuffle_tmp_mask);
> +	list_for_each_entry(stp, &shuffle_task_list, st_l) {
> +		if (!random_shuffle || torture_random(&rand) & 0x1)
> +			set_cpus_allowed_ptr(stp->st_t, shuffle_tmp_mask);
> +	}
>  	mutex_unlock(&shuffle_task_mutex);

Instead of doing it this way, maybe another approach is to randomize the
sleep interval in:

 */
static int torture_shuffle(void *arg)
{
	VERBOSE_TOROUT_STRING("torture_shuffle task started");
	do {
		schedule_timeout_interruptible(shuffle_interval);
		torture_shuffle_tasks();
		...
	} while (...)
	...
}

Right now with this patch you still wakeup the shuffle thread when skipping
the affinity set operation.

thanks,

 - Joel