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SubjectRe: [PATCH] sched/debug: avoid executing show_state and causing rcu stall warning

[ 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:
> */

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