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

[K Prateek Nayak]

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@xxxxxxxxxxx/.

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).

Feel free to include:

Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

> Reported-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-rt-users/20210825132754.GA895675@lothringen/ [1]
> Reported-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-rt-users/xhsmhttqvnall.mognet@xxxxxxxxxxxxxxxxxxx/ [2]
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
> ---
> v3:
>    - get rid of the "link_used" and "ready" flags. They are hard to
>      understand and unnecessary
>    - get rid of the obsolete lockdep_assert_irqs_enabled()
>    - Add lockdep_assert_held(&ep->mtx)
>    - rewrite some comments
> v2:
>    - rename link_locked -> link_used
>    - replace xchg() with smp_store_release() when applicable
>    - make sure llist_node is in clean state when not on a list
>    - remove now-unused list_add_tail_lockless()

--
Thanks and Regards,
Prateek