netdev - RE: [PATCH v4 RESEND] epoll: use refcount to reduce ep

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Date:   Tue, 7 Mar 2023 21:17:39 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Soheil Hassas Yeganeh <soheil@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        "Eric Biggers" <ebiggers@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <brauner@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH v4 RESEND] epoll: use refcount to reduce ep_mutex
 contention



> -----Original Message-----
> From: Paolo Abeni <pabeni@...hat.com>
> Sent: Tuesday, March 7, 2023 10:47 AM
> To: netdev@...r.kernel.org
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>; Al Viro
> <viro@...iv.linux.org.uk>; Carlos Maiolino <cmaiolino@...hat.com>; Eric
> Biggers <ebiggers@...nel.org>; Keller, Jacob E <jacob.e.keller@...el.com>;
> Andrew Morton <akpm@...ux-foundation.org>; Jens Axboe <axboe@...nel.dk>;
> Christian Brauner <brauner@...nel.org>; linux-fsdevel@...r.kernel.org
> Subject: [PATCH v4 RESEND] epoll: use refcount to reduce ep_mutex contention
> 
> We are observing huge contention on the epmutex during an http
> connection/rate test:
> 
>  83.17% 0.25%  nginx            [kernel.kallsyms]         [k]
> entry_SYSCALL_64_after_hwframe
> [...]
>            |--66.96%--__fput
>                       |--60.04%--eventpoll_release_file
>                                  |--58.41%--__mutex_lock.isra.6
>                                            |--56.56%--osq_lock
> 
> The application is multi-threaded, creates a new epoll entry for
> each incoming connection, and does not delete it before the
> connection shutdown - that is, before the connection's fd close().
> 
> Many different threads compete frequently for the epmutex lock,
> affecting the overall performance.
> 
> To reduce the contention this patch introduces explicit reference counting
> for the eventpoll struct. Each registered event acquires a reference,
> and references are released at ep_remove() time.
> 
> Additionally, this introduces a new 'dying' flag to prevent races between
> the EP file close() and the monitored file close().
> ep_eventpoll_release() marks, under f_lock spinlock, each epitem as before
> removing it, while EP file close() does not touch dying epitems.
> 
> The eventpoll struct is released by whoever - among EP file close() and
> and the monitored file close() drops its last reference.
> 
> With all the above in place, we can drop the epmutex usage at disposal time.
> 
> Overall this produces a significant performance improvement in the
> mentioned connection/rate scenario: the mutex operations disappear from
> the topmost offenders in the perf report, and the measured connections/rate
> grows by ~60%.
> 
> To make the change more readable this additionally renames ep_free() to
> ep_clear_and_put(), and moves the actual memory cleanup in a separate
> ep_free() helper.
> 
> Tested-by: Xiumei Mu <xmu@...hiat.com>
> Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
> This is a repost of v4, with no changes. Kindly asking if FS maintainers
> could have a look.

This (still) looks good to me.

Thanks,
Jake