linux-kernel - Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll

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Date:   Wed, 5 Dec 2018 23:46:49 +0000
From:   Eric Wong <e@...24.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce
 ep_poll_callback() contention

Roman Penyaev <rpenyaev@...e.de> wrote:
> Hi all,
> 
> The goal of this patch is to reduce contention of ep_poll_callback() which
> can be called concurrently from different CPUs in case of high events
> rates and many fds per epoll.  Problem can be very well reproduced by
> generating events (write to pipe or eventfd) from many threads, while
> consumer thread does polling.  In other words this patch increases the
> bandwidth of events which can be delivered from sources to the poller by
> adding poll items in a lockless way to the list.

Hi Roman,

I also tried to solve this problem many years ago with help of
the well-tested-in-userspace wfcqueue from Mathieu's URCU.

I was also looking to solve contention with parallel epoll_wait
callers with this.  AFAIK, it worked well; but needed the
userspace tests from wfcqueue ported over to the kernel and more
review.

I didn't have enough computing power to show the real-world
benefits or funding to continue:

	https://lore.kernel.org/lkml/?q=wfcqueue+d:..20130501

It might not be too much trouble for you to brush up the wait-free
patches and test them against the rwlock implementation.

(I only noticed this thread since I was catching up on some
 public-inbox work :>)