Date: Fri, 8 Sep 2023 15:46:48 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>, 
	"davidhwei@...a.com" <davidhwei@...a.com>
Subject: Re: Sockmap's parser/verdict programs and epoll notifications

On Sun, Jul 16, 2023 at 9:37 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Andrii Nakryiko wrote:
> > Hey John,
>
> Sorry missed this while I was on PTO that week.

yeah, vacations tend to cause missing things :)

>
> >
> > We've been recently experimenting with using BPF_SK_SKB_STREAM_PARSER
> > and BPF_SK_SKB_STREAM_VERDICT with sockmap/sockhash to perform
> > in-kernel parsing of RSocket frames. A very simple format ([0]) where
> > the first 3 bytes specify the size of the frame payload. The idea was
> > to collect the entire frame in the kernel before notifying user-space
> > that data is available. This is meant to minimize unnecessary wakeups
> > due to incomplete logical frames, saving CPU.
>
> Nice.
>
> >
> > You can find the BPF source code I've used at [1], it has lots of
> > extra logging and stuff, but the idea is to read the first 3 bytes of
> > each logical frame, and return the expected full frame size from the
> > parser program. The verdict program always just returns SK_PASS.
> >
> > This seems to work exactly as expected in manual simulations of
> > various packet size distributions, and even for a bunch of
> > ping/pong-like benchmark (which are very sensitive to correct frame
> > length determination, so I'm reasonably confident we don't screw that
> > up much). And yet, when benchmarking sending multiple logical RPC
> > streams over the same single socket (so many interleaving RSocket
> > frames on single socket, but in terms of logical frames nothing should
> > change), we often see that while full frame hasn't been accumulated in
> > socket receive buffer yet, epoll_wait() for that socket would return
> > with success notifying user space that there is data on socket.
> > Subsequent recvfrom() call would immediately return -EAGAIN and no
> > data, and our benchmark would go on this loop of useless
> > epoll_wait()+recvfrom() calls back to back, many times over.
>
> Aha yes this sounds bad.
>
> >
> > So I have a few questions:
> >   - is the above use case something that was meant to be handled by
> > sockmap+parser/verdict?
>
> We shouldn't wake up user space if there is nothing to read. So
> yes this seems like a valid use case to me.
>
> >   - is it correct to assume that epoll won't wake up until amount of
> > bytes requested by parser program is accumulated (this seems to be the
> > case from manually experimenting with various "packet delays");
>
> Seems there is some bug that races and causes it to wake up
> user space. I'm aware of a couple bugs in the stream parser
> that I wanted to fix. Not sure I can get to them this week
> but should have time next week. We have a couple more fixes
> to resolve a few HTTPS server compliance tests as well.
>
> >   - is there some known bug or race in how sockmap and strparser
> > framework interacts with epoll subsystem that could cause this weird
> > epoll_wait() behavior?
>
> Yes I know of some races in strparser. I'll elaborate later
> probably with patches as I don't recall them readily at the
> moment.

So I missed a good chunk of BPF mailing list traffic while I was on my
PTO. Did you end up getting to these bugs in strparser logic? Should I
try running the latest bpf-next/net-next on our production workload to
see if this is still happening?

>
> >
> > It does seem like some sort of timing issue, but I couldn't pin down
> > exactly what are the conditions that this happens in. But it's quite
> > reproducible with a pretty high frequency using our internal benchmark
> > when multiple logical streams are involved.
> >
> > Any thoughts or suggestions?
>
> Seems like a bug we should fix it. I'm aware of a couple
> issues with the stream parser that we plan to fix so could
> be one of those or a new one I'm not aware of. I'll take
> a look more closely next week.
>
> >   [0] https://rsocket.io/about/protocol/#framing-format
> >   [1] https://github.com/anakryiko/libbpf-bootstrap/blob/thrift-coalesce-rcvlowat/examples/c/bootstrap.bpf.c
> >
> > -- Andrii

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