netdev - Re: [PATCH] selftests: net: udpgso_bench_tx: Introduce exponential back-off retries

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Date:   Mon, 30 Jan 2023 08:35:16 -0500
From:   Willem de Bruijn <willemb@...gle.com>
To:     Andrei Gherzan <andrei.gherzan@...onical.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Shuah Khan <shuah@...nel.org>, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests: net: udpgso_bench_tx: Introduce exponential
 back-off retries

On Mon, Jan 30, 2023 at 7:51 AM Andrei Gherzan
<andrei.gherzan@...onical.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@...onical.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@...onical.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.