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Miller" , Eric Dumazet , Jakub Kicinski , Shuah Khan , 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 Message-ID: References: <20230127181625.286546-1-andrei.gherzan@canonical.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23/01/30 11:29AM, Willem de Bruijn wrote: > On Mon, Jan 30, 2023 at 11:23 AM Andrei Gherzan > wrote: > > > > On 23/01/30 04:15PM, Andrei Gherzan wrote: > > > On 23/01/30 11:03AM, Willem de Bruijn wrote: > > > > On Mon, Jan 30, 2023 at 9:28 AM Andrei Gherzan > > > > wrote: > > > > > > > > > > On 23/01/30 08:35AM, Willem de Bruijn wrote: > > > > > > On Mon, Jan 30, 2023 at 7:51 AM Andrei Gherzan > > > > > > 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 > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > 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. > > > > Actually wait. I don't think that is the case here. "expected" is the > > number of sends. In this case we sent 1076 more messages than > > completions. Am I missing something obvious? > > Oh indeed. > > Receiving fewer completions than transmission is more likely. Exactly, yes. > This should be the result of datagrams still being somewhere in the > system. In a qdisc, or waiting for the network interface to return a > completion notification, say. > > Does this remain if adding a longer wait before the final flush_errqueue? Yes and no. But not realiably unless I go overboard. > Or, really, the right fix is to keep polling there until the two are > equal, up to some timeout. Currently flush_errqueue calls poll only > once. That makes sense. I have implemented a retry and this ran for a good while now. - flush_errqueue(fd, true); + while (true) { + flush_errqueue(fd, true); + if ((stat_zcopies == num_sends) || (delay >= MAX_DELAY)) + break; + usleep(delay); + delay *= 2; + } What do you think? -- Andrei Gherzan