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

Jan Kiszka
2025-04-09T13:45:02Z

On 09.04.25 14:13, Aaron Lu wrote:
> 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@linutronix.de/
>>
>> 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@bytedance.com/
>>
>> 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@bytedance.com/
>
> Hi Jan,
>
> If you want to give it a try, please try v2.
>

Thanks, we are updating our setup right now.

BTW, does anyone already have a test case that produces the lockup issue
with one or two simple programs and some hectic CFS bandwidth settings?

Jan

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