Re: [PATCH] torture: Fix hang during kthread shutdown phase

[Zhouyi Zhou]

On Sun, Jan 1, 2023 at 2:16 PM Joel Fernandes (Google)
<joel@xxxxxxxxxxxxxxxxx> wrote:
>
> During shutdown of rcutorture, the shutdown thread in
> rcu_torture_cleanup() calls torture_cleanup_begin() which sets fullstop
> to FULLSTOP_RMMOD. This is enough to cause the rcutorture threads for
> readers and fakewriters to breakout of their main while loop and start
> shutting down.
>
> Once out of their main loop, they then call torture_kthread_stopping()
> which in turn waits for kthread_stop() to be called, however
> rcu_torture_cleanup() has not even called kthread_stop() on those
> threads yet, it does that a bit later.  However, before it gets a chance
> to do so, torture_kthread_stopping() calls
> schedule_timeout_interruptible(1) in a tight loop. Tracing confirmed
> this makes the timer softirq constantly execute timer callbacks, while
> never returning back to the softirq exit path and is essentially "locked
> up" because of that. If the softirq preempts the shutdown thread,
> kthread_stop() may never be called.
>
> This commit improves the situation dramatically, by increasing timeout
> passed to schedule_timeout_interruptible() 1/20th of a second. This
> causes the timer softirq to not lock up a CPU and everything works fine.
> Testing has shown 100 runs of TREE07 passing reliably, which was not the
> case before because of RCU stalls.
On my Dell PowerEdge R720 with two Intel(R) Xeon(R) CPU E5-2660 128G memory:
1) before this patch:
3 of 80 rounds failed with "rcu: INFO: rcu_sched detected stalls on
CPUs/tasks" [1]
2) after this patch
all 80 rounds passed

Tested-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>

Thanks
Zhouyi

[1] http://154.220.3.115/logs/20230101/console.log
>
> Cc: Paul McKenney <paulmck@xxxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 6.0.x
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/torture.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/torture.c b/kernel/torture.c
> index 29afc62f2bfe..d024f3b7181f 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -915,7 +915,7 @@ void torture_kthread_stopping(char *title)
>         VERBOSE_TOROUT_STRING(buf);
>         while (!kthread_should_stop()) {
>                 torture_shutdown_absorb(title);
> -               schedule_timeout_uninterruptible(1);
> +               schedule_timeout_uninterruptible(HZ/20);
>         }
>  }
>  EXPORT_SYMBOL_GPL(torture_kthread_stopping);
> --
> 2.39.0.314.g84b9a713c41-goog
>