linux-kernel - Re: Strange issues with epoll since 5.0

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Message-ID: <20190424193903.swlfmfuo6cqnpkwa@dcvr>
Date:   Wed, 24 Apr 2019 19:39:03 +0000
From:   Eric Wong <e@...24.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     linux-kernel@...r.kernel.org, Omar Kilani <omar.kilani@...il.com>
Subject: Re: Strange issues with epoll since 5.0

Omar Kilani <omar.kilani@...il.com> wrote:
> Hi there,
> 
> I’m still trying to piece together a reproducible test that triggers
> this, but I wanted to post in case someone goes “hmmm... change X
> might have done this”.

Maybe Davidlohr knows, since he's responsible for most of the
epoll changes in 5.0.

> Basically, something’s broken (or at least, has changed enough to
> cause problems in user space) in epoll since 5.0. It’s still broken in
> 5.1-rc5.
> 
> It doesn’t happen 100% of the time. It’s sort of hard to pin down but
> I’ve observed the following:
> 
> * nginx not accepting connections under load
> * A java app which uses netty / NIO having strange writability
> semantics on channels, which confuses netty / java enough to not
> properly flush written data on the socket.
> 
> I went and tested these Linux kernels:
> 
> 4.20.17
> 4.19.32
> 4.14.111
> 
> And the issue(s) do not show up there.
> 
> I’m still actively chasing this up, and will report back — I haven’t
> touched kernel code in 15 years so I’m a little rusty. :)
> 
> Regards,
> Omar