Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention

From: Roman Penyaev <rpenyaev@suse.de>
To: Eric Wong <e@80x24.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention
Date: Thu, 06 Dec 2018 11:52:11 +0100	[thread overview]
Message-ID: <39192b9caf1114c95cd23e786a9c3e60@suse.de> (raw)
In-Reply-To: <20181205234649.ssvmv4ulwevgdla4@dcvr>

On 2018-12-06 00:46, Eric Wong wrote:
> Roman Penyaev <rpenyaev@suse.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

Hi Eric,

Nice work.  That was a huge change by itself and by dependency
on wfcqueue.  I could not find any valuable discussion on this,
what was the reaction of the community?

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

Ha :)  I may try to cherry-pick these patches, let's see how many
conflicts I have to resolve, eventpoll.c has been changed a lot
since that (6 years passed, right?)

But reading your work description I can assume that epoll_wait() calls
should be faster, because they do not content with ep_poll_callback(),
and I did not try to solve this, only contention between producers,
which make my change tiny.

I also found your https://yhbt.net/eponeshotmt.c , where you count
number of bare epoll_wait() calls, which IMO is not correct, because
we need to count how many events are delivered, but not how fast
you've returned from epoll_wait().  But as I said no doubts that
getting rid of contention between consumer and producers will show
even better results.

--
Roman