Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller

[Aaron Lu]

On Wed, Apr 09, 2025 at 02:59:18PM +0530, K Prateek Nayak wrote:
> (+ Aaron)

Thank you Prateek for bring me in.

> Hello Jan,
> 
> On 4/9/2025 12:11 PM, Jan Kiszka wrote:
> > On 12.10.23 17:07, Valentin Schneider wrote:
> > > Hi folks,
> > > 
> > > We've had reports of stalls happening on our v6.0-ish frankenkernels, and while
> > > we haven't been able to come out with a reproducer (yet), I don't see anything
> > > upstream that would prevent them from happening.
> > > 
> > > The setup involves eventpoll, CFS bandwidth controller and timer
> > > expiry, and the sequence looks as follows (time-ordered):
> > > 
> > > p_read (on CPUn, CFS with bandwidth controller active)
> > > ======
> > > 
> > > ep_poll_callback()
> > >    read_lock_irqsave()
> > >    ...
> > >    try_to_wake_up() <- enqueue causes an update_curr() + sets need_resched
> > >                        due to having no more runtime
> > >      preempt_enable()
> > >        preempt_schedule() <- switch out due to p_read being now throttled
> > > 
> > > p_write
> > > =======
> > > 
> > > ep_poll()
> > >    write_lock_irq() <- blocks due to having active readers (p_read)
> > > 
> > > ktimers/n
> > > =========
> > > 
> > > timerfd_tmrproc()
> > > `\
> > >    ep_poll_callback()
> > >    `\
> > >      read_lock_irqsave() <- blocks due to having active writer (p_write)
> > > 
> > > 
> > >  From this point we have a circular dependency:
> > > 
> > >    p_read -> ktimers/n (to replenish runtime of p_read)
> > >    ktimers/n -> p_write (to let ktimers/n acquire the readlock)
> > >    p_write -> p_read (to let p_write acquire the writelock)
> > > 
> > > IIUC reverting
> > >    286deb7ec03d ("locking/rwbase: Mitigate indefinite writer starvation")
> > > should unblock this as the ktimers/n thread wouldn't block, but then we're back
> > > to having the indefinite starvation so I wouldn't necessarily call this a win.
> > > 
> > > Two options I'm seeing:
> > > - Prevent p_read from being preempted when it's doing the wakeups under the
> > >    readlock (icky)
> > > - Prevent ktimers / ksoftirqd (*) from running the wakeups that have
> > >    ep_poll_callback() as a wait_queue_entry callback. Punting that to e.g. a
> > >    kworker /should/ do.
> > > 
> > > (*) It's not just timerfd, I've also seen it via net::sock_def_readable -
> > > it should be anything that's pollable.
> > > 
> > > I'm still scratching my head on this, so any suggestions/comments welcome!
> > > 
> > 
> > We are hunting for quite some time sporadic lock-ups or RT systems,
> > first only in the field (sigh), now finally also in the lab. Those have
> > a fairly high overlap with what was described here. Our baselines so
> > far: 6.1-rt, Debian and vanilla. We are currently preparing experiments
> > with latest mainline.
> 
> Do the backtrace from these lockups show tasks (specifically ktimerd)
> waiting on a rwsem? Throttle deferral helps if cfs bandwidth throttling
> becomes the reason for long delay / circular dependency. Is cfs bandwidth
> throttling being used on these systems that run into these lockups?
> Otherwise, your issue might be completely different.

Agree.

> > 
> > While this thread remained silent afterwards, we have found [1][2][3] as
> > apparently related. But this means we are still with this RT bug, even
> > in latest 6.15-rc1?
> 
> I'm pretty sure a bunch of locking related stuff has been reworked to
> accommodate PREEMPT_RT since v6.1. Many rwsem based locking patterns
> have been replaced with alternatives like RCU. Recently introduced
> dl_server infrastructure also helps prevent starvation of fair tasks
> which can allow progress and prevent lockups. I would recommend
> checking if the most recent -rt release can still reproduce your
> issue:
> https://lore.kernel.org/lkml/20250331095610.ulLtPP2C@xxxxxxxxxxxxx/
> 
> Note: Aaron Lu is working on Valentin's approach of deferring cfs
> throttling to exit to user mode boundary
> https://lore.kernel.org/lkml/20250313072030.1032893-1-ziqianlu@xxxxxxxxxxxxx/
> 
> If you still run into the issue of a lockup / long latencies on latest
> -rt release and your system is using cfs bandwidth controls, you can
> perhaps try running with Valentin's or Aaron's series to check if
> throttle deferral helps your scenario.

I just sent out v2 :-)
https://lore.kernel.org/all/20250409120746.635476-1-ziqianlu@xxxxxxxxxxxxx/

Hi Jan,

If you want to give it a try, please try v2.

Thanks.