> -----Original Message----- > From: Paolo Abeni <pabeni@xxxxxxxxxx> > Sent: Tuesday, March 7, 2023 10:47 AM > To: netdev@xxxxxxxxxxxxxxx > Cc: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>; Al Viro > <viro@xxxxxxxxxxxxxxxxxx>; Carlos Maiolino <cmaiolino@xxxxxxxxxx>; Eric > Biggers <ebiggers@xxxxxxxxxx>; Keller, Jacob E <jacob.e.keller@xxxxxxxxx>; > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>; > Christian Brauner <brauner@xxxxxxxxxx>; linux-fsdevel@xxxxxxxxxxxxxxx > 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@xxxxxxxxxxx> > Reviewed-by: Jacob Keller <jacob.e.keller@xxxxxxxxx> > Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx> > Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx> > --- > 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
