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

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Message-ID: <be87a130-62ba-e0a7-1c5b-c48f1e5548e1@redhat.com>
Date:   Tue, 27 Sep 2022 11:26:30 -0400
From:   Waiman Long <longman@...hat.com>
To:     Mukesh Ojha <quic_mojha@...cinc.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

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.

Cheers,
Longman