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From: Eric Wong <e@80x24.org>
To: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Azat Khuzhin <azat@libevent.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 00/14] epoll: support pollable epoll from userspace
Date: Tue, 25 Jun 2019 00:24:56 +0000	[thread overview]
Message-ID: <20190625002456.unhdqihvs5lqcjn6@dcvr> (raw)
In-Reply-To: <20190624144151.22688-1-rpenyaev@suse.de>

Roman Penyaev <rpenyaev@suse.de> wrote:
> Hi all,

+cc Jason Baron
 
> ** Limitations

<snip>

> 4. No support for EPOLLEXCLUSIVE
>      If device does not pass pollflags to wake_up() there is no way to
>      call poll() from the context under spinlock, thus special work is
>      scheduled to offload polling.  In this specific case we can't
>      support exclusive wakeups, because we do not know actual result
>      of scheduled work and have to wake up every waiter.

Lacking EPOLLEXCLUSIVE support is probably a showstopper for
common applications using per-task epoll combined with
non-blocking accept4() (e.g. nginx).


Fwiw, I'm still a weirdo who prefers a dedicated thread doing
blocking accept4 for distribution between tasks (so epoll never
sees a listen socket).  But, depending on what runtime/language
I'm using, I can't always dedicate a blocking thread, so I
recently started using EPOLLEXCLUSIVE from Perl5 where I
couldn't rely on threads being available.


If I could dedicate time to improving epoll; I'd probably
add writev() support for batching epoll_ctl modifications
to reduce syscall traffic, or pick-up the kevent()-like interface
started long ago:
https://lore.kernel.org/lkml/1393206162-18151-1-git-send-email-n1ght.4nd.d4y@gmail.com/
(but I'm not sure I want to increase the size of the syscall table).

  parent reply	other threads:[~2019-06-25  0:24 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-24 14:41 [PATCH v5 00/14] epoll: support pollable epoll from userspace Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 01/14] epoll: move private helpers from a header to the source Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 02/14] epoll: introduce user structures for polling from userspace Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 03/14] epoll: allocate user header and user events ring " Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 04/14] epoll: some sanity flags checks for epoll syscalls " Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 05/14] epoll: offload polling to a work in case of epfd polled " Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 06/14] epoll: introduce helpers for adding/removing events to uring Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 07/14] epoll: call ep_add_event_to_uring() from ep_poll_callback() Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 08/14] epoll: support polling from userspace for ep_insert() Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 09/14] epoll: support polling from userspace for ep_remove() Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 10/14] epoll: support polling from userspace for ep_modify() Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 11/14] epoll: support polling from userspace for ep_poll() Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 12/14] epoll: support mapping for epfd when polled from userspace Roman Penyaev
2019-06-24 14:41 ` [PATCH v5 13/14] epoll: implement epoll_create2() syscall Roman Penyaev
2019-06-24 16:14   ` Arnd Bergmann
2019-06-24 16:25     ` Roman Penyaev
2019-06-25 18:29   ` Eric Wong
2019-06-24 14:41 ` [PATCH v5 14/14] kselftest: add uepoll-test which tests polling from userspace Roman Penyaev
2019-06-24 20:38 ` [PATCH v5 00/14] epoll: support pollable epoll " Linus Torvalds
2019-06-25 11:19   ` Roman Penyaev
2019-06-25  0:24 ` Eric Wong [this message]
2019-06-25 11:07   ` Roman Penyaev

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