Re: [PATCH v2] locking/rtmutex: Limit # of lock stealing for non-RT waiters

[Waiman Long <longman@redhat.com>]

On 6/23/22 09:32, Sebastian Andrzej Siewior wrote:
> On 2022-06-21 15:36:41 [-0400], Waiman Long wrote:
>> Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock
>> stealing") allows unlimited number of lock stealing's for non-RT
>> tasks. That can lead to lock starvation of non-RT top waiter tasks if
>> there is a constant incoming stream of non-RT lockers. This can cause
>> task lockup in PREEMPT_RT kernel. For example,
>>
>> [ 1249.921363] INFO: task systemd:2178 blocked for more than 622 seconds.
>> [ 1872.984225] INFO: task kworker/6:4:63401 blocked for more than 622 seconds.
>>
>> Avoiding this problem and ensuring forward progress by limiting the
>> number of times that a lock can be stolen from each waiter. This patch
>> sets a threshold of 10. That number is arbitrary and can be changed
>> if needed.
>>
>> With that change, the task lockups previously observed when running
>> stressful workloads on PREEMPT_RT kernel disappeared.
> Do you have more insight on how this was tested/ created? Based on that,
> systemd and a random kworker waited on a lock for more than 10 minutes.

The hang happens when our QE team run thier kernel tier 1 test which, I 
think, lasts several hours. The hang happens in some runs but not all of 
them. So it is kind of opportunistic. Mike should be able to provide a 
better idea about frequency and so on.

>
> I added a trace-printk each time a non-RT waiter got the lock stolen,
> kicked a kernel build and a package upgrade and took a look at the stats
> an hour later:
> - sh got its lock stolen 3416 times. I didn't lock the pid so I can't
>    look back and check how long it waited since the first time.
> - the median average of stolen locks is 173.
Maybe we should also more lock stealing per waiter than the 10 that I 
used in the patch. I am open to suggestion to what is a good value to use.
>
>> Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
>> Reported-by: Mike Stowell <mstowell@redhat.com>
>> Signed-off-by: Waiman Long <longman@redhat.com>

Thanks for your time looking at the patch.

Cheers,
Longman