linux-kernel - Re: locking/rwsem: RT throttling issue due to RT task hogging the cpu

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Date:   Tue, 27 Sep 2022 21:00:47 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, <mingo@...hat.com>,
        <will@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <"<boqun.feng"@gmail.com>
Subject: Re: locking/rwsem: RT throttling issue due to RT task hogging the cpu

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
>