Date:   Mon, 30 Jan 2023 16:15:43 +0000
From:   Andrei Gherzan <andrei.gherzan@canonical.com>
To:     Willem de Bruijn <willemb@google.com>
Cc:     Paolo Abeni <pabeni@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Shuah Khan <shuah@kernel.org>, netdev@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] selftests: net: udpgso_bench_tx: Introduce exponential
 back-off retries
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On 23/01/30 11:03AM, Willem de Bruijn wrote:
> On Mon, Jan 30, 2023 at 9:28 AM Andrei Gherzan
> <andrei.gherzan@canonical.com> wrote:
> >
> > On 23/01/30 08:35AM, Willem de Bruijn wrote:
> > > On Mon, Jan 30, 2023 at 7:51 AM Andrei Gherzan
> > > <andrei.gherzan@canonical.com> wrote:
> > > >
> > > > On 23/01/30 09:26AM, Paolo Abeni wrote:
> > > > > On Fri, 2023-01-27 at 17:03 -0500, Willem de Bruijn wrote:
> > > > > > On Fri, Jan 27, 2023 at 1:16 PM Andrei Gherzan
> > > > > > <andrei.gherzan@canonical.com> wrote:
> > > > > > >
> > > > > > > The tx and rx test programs are used in a couple of test scripts including
> > > > > > > "udpgro_bench.sh". Taking this as an example, when the rx/tx programs
> > > > > > > are invoked subsequently, there is a chance that the rx one is not ready to
> > > > > > > accept socket connections. This racing bug could fail the test with at
> > > > > > > least one of the following:
> > > > > > >
> > > > > > > ./udpgso_bench_tx: connect: Connection refused
> > > > > > > ./udpgso_bench_tx: sendmsg: Connection refused
> > > > > > > ./udpgso_bench_tx: write: Connection refused
> > > > > > >
> > > > > > > This change addresses this by adding routines that retry the socket
> > > > > > > operations with an exponential back off algorithm from 100ms to 2s.
> > > > > > >
> > > > > > > Fixes: 3a687bef148d ("selftests: udp gso benchmark")
> > > > > > > Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
> > > > > >
> > > > > > Synchronizing the two processes is indeed tricky.
> > > > > >
> > > > > > Perhaps more robust is opening an initial TCP connection, with
> > > > > > SO_RCVTIMEO to bound the waiting time. That covers all tests in one
> > > > > > go.
> > > > >
> > > > > Another option would be waiting for the listener(tcp)/receiver(udp)
> > > > > socket to show up in 'ss' output before firing-up the client - quite
> > > > > alike what mptcp self-tests are doing.
> > > >
> > > > I like this idea. I have tested it and it works as expected with the
> > > > exeception of:
> > > >
> > > > ./udpgso_bench_tx: sendmsg: No buffer space available
> > > >
> > > > Any ideas on how to handle this? I could retry and that works.
> > >
> > > This happens (also) without the zerocopy flag, right? That
> > >
> > > It might mean reaching the sndbuf limit, which can be adjusted with
> > > SO_SNDBUF (or SO_SNDBUFFORCE if CAP_NET_ADMIN). Though I would not
> > > expect this test to bump up against that limit.
> > >
> > > A few zerocopy specific reasons are captured in
> > > https://www.kernel.org/doc/html/latest/networking/msg_zerocopy.html#transmission.
> >
> > I have dug a bit more into this, and it does look like your hint was in
> > the right direction. The fails I'm seeing are only with the zerocopy
> > flag.
> >
> > From the reasons (doc) above I can only assume optmem limit as I've
> > reproduced it with unlimited locked pages and the fails are transient.
> > That leaves optmem limit. Bumping the value I have by default (20480) to
> > (2048000) made the sendmsg succeed as expected. On the other hand, the
> > tests started to fail with something like:
> >
> > ./udpgso_bench_tx: Unexpected number of Zerocopy completions:    774783
> > expected    773707 received
> 
> More zerocopy completions than number of sends. I have not seen this before.
> 
> The completions are ranges of IDs, one per send call for datagram sockets.
> 
> Even with segmentation offload, the counter increases per call, not per segment.
> 
> Do you experience this without any other changes to udpgso_bench_tx.c.
> Or are there perhaps additional sendmsg calls somewhere (during
> initial sync) that are not accounted to num_sends?

Indeed, that looks off. No, I have run into this without any changes in
the tests (besides the retry routine in the shell script that waits for
rx to come up). Also, as a data point.

As an additional data point, this was only seen on the IPv6 tests. I've
never been able to replicate it on the IPv4 run.

-- 
Andrei Gherzan