netdev - Re: [PATCH v2] epoll: use refcount to reduce ep

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Date:   Fri, 25 Nov 2022 08:37:43 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jason Baron <jbaron@...mai.com>,
        Roman Penyaev <rpenyaev@...e.de>, netdev@...r.kernel.org,
        Carlos Maiolino <cmaiolino@...hat.com>
Subject: Re: [PATCH v2] epoll: use refcount to reduce ep_mutex contention

Hello,

On Thu, 2022-11-24 at 15:02 -0800, Eric Biggers wrote:
> On Thu, Nov 24, 2022 at 06:57:41PM +0100, Paolo Abeni wrote:
> > 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. ep_free() doesn't touch
> > anymore the event RB tree, it just unregisters the existing callbacks
> > and drops a reference to the ep struct. The struct itself is freed when
> > the reference count reaches 0. The reference count updates are protected
> > by the mtx mutex so no additional atomic operations are needed.
> 
> So, the behavior before this patch is that closing an epoll file frees all
> resources associated with it.  This behavior is documented in the man page
> epoll_create(2): "When all file descriptors referring to an epoll instance have
> been closed, the kernel destroys the instance and releases the associated
> resources for reuse."
> 
> The behavior after this patch is that the resources aren't freed until the epoll
> file *and* all files that were added to it have been closed.
> 
> Is that okay? 

This is actually the question that I intended to raise here. I should
have probably make it explicit. 

Also thank you for pointing out the man page info, at very least this
patch would require updating it - or possibly that is a reason to shot
this patch completelly. I would love to ear more opinions ;)

>  I suppose in most cases it is, since the usual use case for epoll
> is to have a long-lived epoll instance and shorter lived file descriptors that
> are polled using that long-lived epoll instance.
> 
> But probably some users do things the other way around.  I.e., they have a
> long-lived file descriptor that is repeatedly polled using different epoll
> instances that have a shorter lifetime.
> 
> In that case, the number of 'struct eventpoll' and 'struct epitem' in kernel
> memory will keep growing until 'max_user_watches' is hit, at which point
> EPOLL_CTL_ADD will start failing with ENOSPC.
> 
> Are you sure that is fine?

I did not think about such use-case, thank you for pointing that out!
Even if it looks like quite a corner-case, it also looks like quite a
deal breaker to me. Again other opinions more then welcome! ;)

Please allow me a question: do you think that solving the contention
problem reported here inside the kernel is worthy? Or should we
encourage (or enforce) the user-space to always do EPOLL_CTL_DEL for
better scalability?

Thanks,

Paolo