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

[Joel Fernandes]

> On Jan 1, 2023, at 8:02 AM, Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> wrote:
> 
> 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! Glad to see your tests look good now.

 - Joel


> 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
>>