Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT waiters

[Waiman Long <longman@redhat.com>]

On 7/7/22 15:04, Boqun Feng wrote:
> On Thu, Jul 07, 2022 at 02:45:10PM -0400, Waiman Long wrote:
>> On 7/7/22 14:22, Boqun Feng wrote:
>>> On Wed, Jul 06, 2022 at 10:03:10AM -0400, Waiman Long wrote:
>>>> On 7/6/22 09:59, 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
>>>>> rcu_preempt self-detected stall or even task lockup in PREEMPT_RT kernel.
>>>>> For example,
>>>>>
>>>>> [77107.424943] rcu: INFO: rcu_preempt self-detected stall on CPU
>>>>> [ 1249.921363] INFO: task systemd:2178 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 32. That number is arbitrary and can be changed
>>>>> if needed.
>>>>>
>>>>> Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
>>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>>> ---
>>>>>     kernel/locking/rtmutex.c        | 9 ++++++---
>>>>>     kernel/locking/rtmutex_common.h | 8 ++++++++
>>>>>     2 files changed, 14 insertions(+), 3 deletions(-)
>>>>>
>>>>>     [v3: Increase threshold to 32 and add rcu_preempt self-detected stall]
>>>> Note that I decided to increase the threshold to 32 from 10 to reduce the
>>>> potential performance impact of this change, if any. We also found out that
>>>> this patch can fix some of the rcu_preempt self-detected stall problems that
>>>> we saw with the PREEMPT_RT kernel. So I added that information in the patch
>>>> description.
>>>>
>>> Have you considered (and tested) whether we can set the threshold
>>> directly proportional to nr_cpu_ids? Because IIUC, the favorable case
>>> for lock stealing is that every CPU gets a chance to steal once. If one
>>> CPU can steal twice, 1) either there is a context switch between two
>>> tasks, which costs similarly as waking up the waiter, or 2) a task drops
>>> and re-graps a lock, which means the task wants to yield to other
>>> waiters of the lock.
>> There is no inherent restriction on not allowing the same cpu stealing the
>> lock twice or more. With rtmutex, the top waiter may be sleeping and the
> Well, I'm not saying we need to restrict the same cpu to steal a lock
> twice or more. Think about this, when there is a task running on CPU 1
> already steals a lock once, for example:
>
> 	<lock release>
> 	{task C is the top waiter}
>
> 	CPU 1
> 	=====
> 	<now task A running>
> 	lock(); // steal the lock
> 	...
> 	unlock():
> 	  // set owner to NULL
> 	  <switch task B> // similar cost to wake up A
> 	  lock(); // steal the lock
>
> , which means if a CPU steals a lock twice or more, it's almost certain
> that a context happened between two steals ("almost" because there could
> be a case where task A lock()+unlock() twice, but as I said, it
> means that task A is willing to yield.).
>
> Therefore if there are @nr_cpu_ids lock steals, it means either there is
> a context switch somewhere or a task has been willing to yield. And I
> think it's a reasonable signal to stop lock stealing.
>
> Thoughts?

The reality is that a task can acquire the same lock multiple times 
before a context switch. So I believe stealing a lock from the same 
sleeping top waiter multiple times can certainly happen. For a large SMP 
systems with hundred or even thousands of cpus, allowing that many lock 
stealing may significantly increase the lock acquisition latency for the 
unfortunate tasks.

Another alternative that I have done in the past is to put in a time 
stamp where a task become the top waiter and refrained from stealing the 
lock when the elapsed time from the time stamp exceeds a certain limit. 
That will limit the max lock acquisition latency of a non-RT task as 
long as there are no other RT tasks competing with it for the lock.

Cheers,
Longman