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SubjectRe: locking/rwsem: RT throttling issue due to RT task hogging the cpu
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Hi Waiman,

Thanks for the reply.

On 9/27/2022 8:56 PM, Waiman Long wrote:
> On 9/27/22 11:25, Waiman Long wrote:
>>
>> On 9/20/22 12:19, Mukesh Ojha wrote:
>>> Hi,
>>>
>>> We are observing one issue where, sem->owner is not set and
>>> sem->count=6 [1] which means both RWSEM_FLAG_WAITERS and
>>> RWSEM_FLAG_HANDOFF bits are set. And if unfold the sem->wait_list we
>>> see the following order of process waiting [2] where [a] is waiting
>>> for write, while [b],[c] are waiting for read and [d] is the RT task
>>> for which waiter.handoff_set=true and it is continuously running on
>>> cpu7 and not letting the first write waiter [a] on cpu7.
>>>
>>> [1]
>>>
>>>   sem = 0xFFFFFFD57DDC6680 -> (
>>>     count = (counter = 6),
>>>     owner = (counter = 0),
>>>
>>> [2]
>>>
>>> [a] kworker/7:0 pid: 32516 ==> [b] iptables-restor pid: 18625 ==>
>>> [c]HwBinder:1544_3  pid: 2024 ==> [d] RenderEngine pid: 2032 cpu: 7
>>> prio:97 (RT task)
>>>
>>>
>>> Sometime back, Waiman has suggested this which could help in RT task
>>> leaving the cpu.
>>>
>>> https://lore.kernel.org/all/8c33f989-8870-08c6-db12-521de634b34e@redhat.com/
>>>
>>>
>> Sorry for the late reply. There is now an alternative way of dealing
>> with this RT task hogging issue with the commit 48dfb5d2560d
>> ("locking/rwsem: Disable preemption while trying for rwsem lock").
>> Could you try it to see if it can address your problem?
>
> FYI, this commit is in the tip tree. It is not in the mainline yet.


I only posted that patch so, i am aware about it. In that issue
sem->count was 7 and here it is 6 and current issue occurs after fix
48dfb5d2560d ("locking/rwsem: Disable preemption while trying for rwsem
lock").

-Mukesh

>
> Cheers,
> Longman
>

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