Re: Strange issues with epoll since 5.0

[Davidlohr Bueso]

On Wed, 24 Apr 2019, Eric Wong wrote:

> Omar Kilani <omar.kilani@xxxxxxxxx> 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.

Not really, I have not been made aware of any issues until now.

> > 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. :)

A bisection and/or workload that triggers the issue would be great to
see what's going on.

Thanks,
Davidlohr