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

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Date:   Mon, 30 Jan 2023 14:28:08 +0000
From:   Andrei Gherzan <andrei.gherzan@...onical.com>
To:     Willem de Bruijn <willemb@...gle.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 23/01/30 08:35AM, Willem de Bruijn wrote:
> 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.

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

Also, this audit fail is transient as with the buffer limit one.

-- 
Andrei Gherzan