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SubjectRe: locking/rwsem: RT throttling issue due to RT task hogging the cpu
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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?

Thanks,
Longman

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