Re: [PATCH v3] eventpoll: Fix priority inversion problem

[Florian Bezdeka <florian.bezdeka@siemens.com>]

On Mon, 2025-06-30 at 20:38 +0530, K Prateek Nayak wrote:
> Hello Nam,
> 
> On 5/27/2025 2:38 PM, Nam Cao wrote:
> > The ready event list of an epoll object is protected by read-write
> > semaphore:
> > 
> >    - The consumer (waiter) acquires the write lock and takes items.
> >    - the producer (waker) takes the read lock and adds items.
> > 
> > The point of this design is enabling epoll to scale well with large number
> > of producers, as multiple producers can hold the read lock at the same
> > time.
> > 
> > Unfortunately, this implementation may cause scheduling priority inversion
> > problem. Suppose the consumer has higher scheduling priority than the
> > producer. The consumer needs to acquire the write lock, but may be blocked
> > by the producer holding the read lock. Since read-write semaphore does not
> > support priority-boosting for the readers (even with CONFIG_PREEMPT_RT=y),
> > we have a case of priority inversion: a higher priority consumer is blocked
> > by a lower priority producer. This problem was reported in [1].
> > 
> > Furthermore, this could also cause stall problem, as described in [2].
> > 
> > To fix this problem, make the event list half-lockless:
> > 
> >    - The consumer acquires a mutex (ep->mtx) and takes items.
> >    - The producer locklessly adds items to the list.
> > 
> > Performance is not the main goal of this patch, but as the producer now can
> > add items without waiting for consumer to release the lock, performance
> > improvement is observed using the stress test from
> > https://github.com/rouming/test-tools/blob/master/stress-epoll.c. This is
> > the same test that justified using read-write semaphore in the past.
> > 
> > Testing using 12 x86_64 CPUs:
> > 
> >            Before     After        Diff
> > threads  events/ms  events/ms
> >        8       6932      19753    +185%
> >       16       7820      27923    +257%
> >       32       7648      35164    +360%
> >       64       9677      37780    +290%
> >      128      11166      38174    +242%
> > 
> > Testing using 1 riscv64 CPU (averaged over 10 runs, as the numbers are
> > noisy):
> > 
> >            Before     After        Diff
> > threads  events/ms  events/ms
> >        1         73        129     +77%
> >        2        151        216     +43%
> >        4        216        364     +69%
> >        8        234        382     +63%
> >       16        251        392     +56%
> > 
> 
> I gave this patch a spin on top of tip:sched/core (PREEMPT_RT) with
> Jan's reproducer from
> https://lore.kernel.org/all/7483d3ae-5846-4067-b9f7-390a614ba408@siemens.com/.
> 
> On tip:sched/core, I see a hang few seconds into the run and rcu-stall
> a minute after when I pin the epoll-stall and epoll-stall-writer on the
> same CPU as the Bandwidth timer on a 2vCPU VM. (I'm using a printk to
> log the CPU where the timer was started in pinned mode)
> 
> With this series, I haven't seen any stalls yet over multiple short
> runs (~10min) and even a longer run (~3Hrs).

Many thanks for running those tests and posting the results as comments
to this series. Highly appreciated!

Florian