Re: [PATCH v4 1/1] eventpoll: Replace rwlock with spinlock

[Nam Cao <namcao@linutronix.de>]

On Tue, Jul 15, 2025 at 02:46:34PM +0200, 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].
> 
> Fix this problem by replacing rwlock with spinlock.
> 
> This reduces the event bandwidth, as the producers now have to contend with
> each other for the spinlock. According to the benchmark from
> https://github.com/rouming/test-tools/blob/master/stress-epoll.c:
> 
>     On 12 x86 CPUs:
>                   Before     After        Diff
>         threads  events/ms  events/ms
>               8       7162       4956     -31%
>              16       8733       5383     -38%
>              32       7968       5572     -30%
>              64      10652       5739     -46%
>             128      11236       5931     -47%
> 
>     On 4 riscv CPUs:
>                   Before     After        Diff
>         threads  events/ms  events/ms
>               8       2958       2833      -4%
>              16       3323       3097      -7%
>              32       3451       3240      -6%
>              64       3554       3178     -11%
>             128       3601       3235     -10%
> 
> Although the numbers look bad, it should be noted that this benchmark
> creates multiple threads who do nothing except constantly generating new
> epoll events, thus contention on the spinlock is high. For real workload,
> the event rate is likely much lower, and the performance drop is not as
> bad.
> 
> Using another benchmark (perf bench epoll wait) where spinlock contention
> is lower, improvement is even observed on x86:
> 
>     On 12 x86 CPUs:
>         Before: Averaged 110279 operations/sec (+- 1.09%), total secs = 8
>         After:  Averaged 114577 operations/sec (+- 2.25%), total secs = 8
> 
>     On 4 riscv CPUs:
>         Before: Averaged 175767 operations/sec (+- 0.62%), total secs = 8
>         After:  Averaged 167396 operations/sec (+- 0.23%), total secs = 8
> 
> In conclusion, no one is likely to be upset over this change. After all,
> spinlock was used originally for years, and the commit which converted to
> rwlock didn't mention a real workload, just that the benchmark numbers are
> nice.
> 
> This patch is not exactly the revert of commit a218cc491420 ("epoll: use
> rwlock in order to reduce ep_poll_callback() contention"), because git
> revert conflicts in some places which are not obvious on the resolution.
> This patch is intended to be backported, therefore go with the obvious
> approach:
> 
>   - Replace rwlock_t with spinlock_t one to one
> 
>   - Delete list_add_tail_lockless() and chain_epi_lockless(). These were
>     introduced to allow producers to concurrently add items to the list.
>     But now that spinlock no longer allows producers to touch the event
>     list concurrently, these two functions are not necessary anymore.
> 
> Fixes: a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention")
> Signed-off-by: Nam Cao <namcao@linutronix.de>
> Cc: stable@vger.kernel.org

I forgot to add:

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Closes: https://lore.kernel.org/linux-rt-users/20210825132754.GA895675@lothringen/ [1]
Reported-by: Valentin Schneider <vschneid@redhat.com>
Closes: https://lore.kernel.org/linux-rt-users/xhsmhttqvnall.mognet@vschneid.remote.csb/ [2]

Christian, do you mind adding those for me, if/when you apply the patch?

Nam