Messages in this thread | | | Date | Thu, 23 Jun 2022 10:41:17 -0400 | Subject | Re: [PATCH v2] locking/rtmutex: Limit # of lock stealing for non-RT waiters | From | Waiman Long <> |
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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
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