Re: [PATCH] sched/debug: Reset watchdog on all CPUs while processing sysrq-t

["liwei (GF)" <liwei391@huawei.com>]

Hi Steven,
Yes, it can be triggered on the Hi1620 system (128 cores) as follows:
stress-ng -c 50 &
stress-ng -m 50 &
stress-ng -i 20 &
echo 7 > /proc/sys/kernel/printk
echo t > /proc/sysrq-trigger

Then a soft lockup will be reported at migration thread
[39636.303531] watchdog: BUG: soft lockup - CPU#67 stuck for 23s! [migration/67:348]
which is waiting for the CPU handling sysrq-t to process stop_two_cpus.

Thanks,
Wei

On 2020/1/3 3:45, Steven Rostedt wrote:
> On Thu, 26 Dec 2019 16:52:24 +0800
> Wei Li <liwei391@huawei.com> wrote:
> 
>> Lengthy output of sysrq-t may take a lot of time on slow serial console
>> with lots of processes and CPUs.
>>
>> So we need to reset NMI-watchdog to avoid spurious lockup messages, and
>> we also reset softlockup watchdogs on all other CPUs since another CPU
>> might be blocked waiting for us to process an IPI or stop_machine.
> 
> Have you had this triggered?
> 
>>
>> Add to sysrq_sched_debug_show() as what we did in show_state_filter().
>>
>> Signed-off-by: Wei Li <liwei391@huawei.com>
>> ---
>>  kernel/sched/debug.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>> index f7e4579e746c..879d3ccf3806 100644
>> --- a/kernel/sched/debug.c
>> +++ b/kernel/sched/debug.c
>> @@ -751,9 +751,16 @@ void sysrq_sched_debug_show(void)
>>  	int cpu;
>>  
>>  	sched_debug_header(NULL);
>> -	for_each_online_cpu(cpu)
>> +	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();
> 
> This doesn't seem to hurt to add, thus.
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> 
> -- Steve
> 
>>  		print_cpu(NULL, cpu);
>> -
>> +	}
>>  }
>>  
>>  /*
> 
> 
> .
>