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

[Waiman Long]

On 9/27/22 11:30, Mukesh Ojha wrote:
> 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@xxxxxxxxxx/ 
> > > 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").

Thanks for the quick reply. So it doesn't completely fix this RT hogging 
issue. It is harder than I thought. Will look further into this.

Cheers,
Longman