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Paolo Abeni authored
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. The eventpoll struct is released by whoever - among EP file close() and and the monitored file close() drops its last reference. 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 dying before removing it, while EP file close() does not touch dying epitems. The above is needed as both close operations could run concurrently and drop the EP reference acquired via the epitem entry. Without the above flag, the monitored file close() could reach the EP struct via the epitem list while the epitem is still listed and then try to put it after its disposal. An alternative could be avoiding touching the references acquired via the epitems at EP file close() time, but that could leave the EP struct alive for potentially unlimited time after EP file close(), with nasty side effects. 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. Link: https://lkml.kernel.org/r/4a57788dcaf28f5eb4f8dfddcc3a8b172a7357bb.1679504153.git.pabeni@redhat.comSigned-off-by:
Paolo Abeni <pabeni@redhat.com> Co-developed-by:
Eric Dumazet <edumazet@google.com> Signed-off-by:
Eric Dumazet <edumazet@google.com> Tested-by:
Xiumei Mu <xmu@redhiat.com> Acked-by:
Soheil Hassas Yeganeh <soheil@google.com> Reviewed-by:
Davidlohr Bueso <dave@stgolabs.net> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Carlos Maiolino <cmaiolino@redhat.com> Cc: Christian Brauner <brauner@kernel.org> Cc: Eric Biggers <ebiggers@kernel.org> Cc: Jacob Keller <jacob.e.keller@intel.com> Cc: Jens Axboe <axboe@kernel.dk> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org>
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