linux-kernel - Re: [PATCH] epoll: remove ep_call_nested() from ep_eventpoll

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Date:   Wed, 1 Nov 2017 13:53:54 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Jason Baron <jbaron@...mai.com>, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Salman Qazi <sqazi@...gle.com>, Hou Tao <houtao1@...wei.com>
Subject: Re: [PATCH] epoll: remove ep_call_nested() from ep_eventpoll_poll()

On Tue, 31 Oct 2017 07:58:21 -0700 Davidlohr Bueso <dave@...olabs.net> wrote:

> On Tue, 31 Oct 2017, Jason Baron wrote:
> 
> >The use of ep_call_nested() in ep_eventpoll_poll(), which is the .poll
> >routine for an epoll fd, is used to prevent excessively deep epoll
> >nesting, and to prevent circular paths. However, we are already preventing
> >these conditions during EPOLL_CTL_ADD. In terms of too deep epoll chains,
> >we do in fact allow deep nesting of the epoll fds themselves (deeper
> >than EP_MAX_NESTS), however we don't allow more than EP_MAX_NESTS when
> >an epoll file descriptor is actually connected to a wakeup source. Thus,
> >we do not require the use of ep_call_nested(), since ep_eventpoll_poll(),
> >which is called via ep_scan_ready_list() only continues nesting if there
> >are events available. Since ep_call_nested() is implemented using a global
> >lock, applications that make use of nested epoll can see large performance
> >improvements with this change.
> 
> Improvements are quite obscene actually, such as for the following epoll_wait()
> benchmark with 2 level nesting on a 80 core IvyBridge:
> 
> ncpus  vanilla     dirty     delta
> 1      2447092     3028315   +23.75%
> 4      231265      2986954   +1191.57%
> 8      121631      2898796   +2283.27%
> 16     59749       2902056   +4757.07%
> 32     26837	   2326314   +8568.30%
> 64     12926       1341281   +10276.61%
> 
> (http://linux-scalability.org/epoll/epoll-test.c)

This is tempting, but boy it is late in the -rc cycle.

How important are these workloads?  Would the world end if we held off
on this for 4.15?