Re: [PATCH 0/6] RFC: Clean up tap-side event handling

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 5 Sep 2024 10:32:38 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Thu, 5 Sep 2024 10:35:14 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Wed, Sep 04, 2024 at 07:19:22PM +0200, Stefano Brivio wrote:  
> > > On Wed, 4 Sep 2024 13:17:53 +1000
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >     
> > > > On Tue, Sep 03, 2024 at 09:25:54PM +0200, Stefano Brivio wrote:    
> > > > > On Tue,  3 Sep 2024 22:02:29 +1000
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >       
> > > > > > This is a draft patch working towards adding EPOLLOUT handling to the
> > > > > > tap code, which could then be used to "unstick" flows which have
> > > > > > unsent data from the socket side.  For now that's just a stub, but
> > > > > > makes what I think are some worthwhile cleanups to the tap side event
> > > > > > handling in the meantime.      
> > > > > 
> > > > > Except for the issue in 3/6 and nits elsewhere, it all makes sense and
> > > > > tap-side EPOLLOUT handling is definitely going to be an improvement.
> > > > > 
> > > > > I wonder if it's the right moment for this kind of series, though, in
> > > > > terms of future bisections, as long as we're grappling with
> > > > > https://github.com/containers/podman/issues/23686 and
> > > > > https://bugs.passt.top/show_bug.cgi?id=94. Assuming, of course, that
> > > > > this series doesn't fix anything.      
> > > > 
> > > > I don't think this series will fix anything as it stands.  It is,
> > > > indirectly, aimed at addressing bug 94.  I'm struggling to figure out
> > > > what to do with bug 94, because I find it almost impossible to reason
> > > > about the current event masks in TCP.    
> > > 
> > > I don't see at the moment anything indicating TCP issues other than the
> > > one you addressed with your tentative debug patch at:
> > > 
> > >   https://passt.top/passt/commit/?h=podman23686&id=026fb71d1dde60135d95741552906fd5320384bc
> > > 
> > > Given that, with that patch, we had at least another report of event
> > > storms, this time on UDP, that is, the one from:
> > > 
> > >   https://github.com/containers/podman/issues/23686#issuecomment-2324945010
> > > 
> > > I shared this other one on top:
> > > 
> > >   https://passt.top/passt/commit/?h=podman23686&id=0c6c20dee5c24bd324834a99f409ad43c50812ae    
> > 
> > Ah, nice.
> >   
> > > > I'd really like to simplify
> > > > them so it's clearer what's correct and not and I think the most
> > > > obvious path to doing so is using EPOLLET all the time.  That requires
> > > > some sort of kick when the tap is ready to accept more data, hence
> > > > this series as a prerequisite.    
> > > 
> > > Sure, it's going to be simpler and more robust, but on the other hand
> > > we wouldn't notice these kind of issues.    
> > 
> > Uh.. I'm confused.  In what way would we not notice issues, other than
> > the issues not existing which.. would be good, right?  
> 
> Right now, we have a condition where we fail to handle EPOLLRDHUP
> before an outbound connection is established, see
> https://github.com/containers/podman/issues/23686#issuecomment-233023742,

Sorry, it's:

  https://github.com/containers/podman/issues/23686#issuecomment-2330237424

> and we end up in a tight event processing loop.
> 
> I guess what we're missing in tcp_sock_handler() is clear (we should
> orderly close the connection), but the tight loop didn't happen on
> 2024_06_24.1ee2eca (I'm bisecting right now) and we don't know why it
> didn't.
> 
> If we set EPOLLET, we won't see that anymore, because the EPOLLRDHUP
> event is reported just once, but that doesn't mean we solved this.

-- 
Stefano