linux-kernel - Re: [PATCH v5 00/14] epoll: support pollable epoll from userspace

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Date:   Tue, 25 Jun 2019 00:24:56 +0000
From:   Eric Wong <e@...24.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Azat Khuzhin <azat@...event.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 00/14] epoll: support pollable epoll from userspace

Roman Penyaev <rpenyaev@...e.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).