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Date:   Wed, 3 Aug 2022 08:42:35 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Liu Song <liusong@linux.alibaba.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        bsegall@google.com, mgorman@suse.de, bristot@redhat.com,
        vschneid@redhat.com, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu
 stall warning
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[ Adding Paul ]

On Wed,  3 Aug 2022 09:18:45 +0800
Liu Song <liusong@linux.alibaba.com> wrote:

> From: Liu Song <liusong@linux.alibaba.com>
> 
> If the number of CPUs is large, "sysrq_sched_debug_show" will execute for
> a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my
> 128-core machine, the rcu stall warning will be triggered. Moreover,
> sysrq_sched_debug_show does not need to be protected by rcu_read_lock,
> and no rcu stall warning will appear after adjustment.
> 
> Signed-off-by: Liu Song <liusong@linux.alibaba.com>
> ---
>  kernel/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5555e49..82c117e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8879,11 +8879,11 @@ void show_state_filter(unsigned int state_filter)
>  			sched_show_task(p);
>  	}
>  
> +	rcu_read_unlock();
>  #ifdef CONFIG_SCHED_DEBUG
>  	if (!state_filter)
>  		sysrq_sched_debug_show();

If this is just because sysrq_sched_debug_show() is very slow, does RCU
have a way to "touch" it? Like the watchdogs have? That is, to tell RCU
"Yes I know I'm taking a long time, but I'm still making forward progress,
don't complain about me". Then the sysrq_sched_debug_show() could have:

	for_each_online_cpu(cpu) {
		/*
		 * Need to reset softlockup watchdogs on all CPUs, because
		 * another CPU might be blocked waiting for us to process
		 * an IPI or stop_machine.
		 */
		touch_nmi_watchdog();
		touch_all_softlockup_watchdogs();
+		touch_rcu();
		print_cpu(NULL, cpu);
	}

??

-- Steve

>  #endif
> -	rcu_read_unlock();
>  	/*
>  	 * Only show locks if all tasks are dumped:
>  	 */