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Date:   Fri, 27 Sep 2019 19:29:15 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     hev <r@hev.cc>
Cc:     linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        Davide Libenzi <davidel@xmailserver.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Eric Wong <e@80x24.org>, Jason Baron <jbaron@akamai.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Roman Penyaev <rpenyaev@suse.de>,
        Sridhar Samudrala <sridhar.samudrala@intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND v4] fs/epoll: Remove unnecessary wakeups of
 nested epoll that in ET mode
Message-Id: <20190927192915.6ec24ad706258de99470a96e@linux-foundation.org>
In-Reply-To: <20190925015603.10939-1-r@hev.cc>
References: <20190925015603.10939-1-r@hev.cc>
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On Wed, 25 Sep 2019 09:56:03 +0800 hev <r@hev.cc> wrote:

> From: Heiher <r@hev.cc>
> 
> Take the case where we have:
> 
>         t0
>          | (ew)
>         e0
>          | (et)
>         e1
>          | (lt)
>         s0
> 
> t0: thread 0
> e0: epoll fd 0
> e1: epoll fd 1
> s0: socket fd 0
> ew: epoll_wait
> et: edge-trigger
> lt: level-trigger
> 
> We only need to wakeup nested epoll fds if something has been queued to the
> overflow list, since the ep_poll() traverses the rdllist during recursive poll
> and thus events on the overflow list may not be visible yet.
> 
> Test code:

Look sane to me.  Do you have any performance testing results which
show a benefit?

epoll maintainership isn't exactly a hive of activity nowadays :(
Roman, would you please have time to review this?