netdev - Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy

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Date:   Thu, 17 Nov 2022 17:38:01 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
        David Ahern <dsahern@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>, rcu@...r.kernel.org,
        rostedt@...dmis.org, paulmck@...nel.org, fweisbec@...il.com,
        jiejiang@...gle.com, Thomas Glexiner <tglx@...utronix.de>
Subject: Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy_rcu

On Thu, Nov 17, 2022 at 5:17 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Nov 17, 2022 at 7:58 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > Hello Eric,
> >
> > On Wed, Nov 16, 2022 at 07:44:41PM -0800, Eric Dumazet wrote:
> > > On Wed, Nov 16, 2022 at 7:16 PM Joel Fernandes (Google)
> > > <joel@...lfernandes.org> wrote:
> > > >
> > > > In a networking test on ChromeOS, we find that using the new CONFIG_RCU_LAZY
> > > > causes a networking test to fail in the teardown phase.
> > > >
> > > > The failure happens during: ip netns del <name>
> > >
> > > And ? What happens then next ?
> >
> > The test is doing the 'ip netns del <name>' and then polling for the
> > disappearance of a network interface name for upto 5 seconds. I believe it is
> > using netlink to get a table of interfaces. That polling is timing out.
> >
> > Here is some more details from the test's owner (copy pasting from another
> > bug report):
> > In the cleanup, we remove the netns, and thus will cause the veth pair being
> > removed automatically, so we use a poll to check that if the veth in the root
> > netns still exists to know whether the cleanup is done.
> >
> > Here is a public link to the code that is failing (its in golang):
> > https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/tast-tests/src/chromiumos/tast/local/network/virtualnet/env/env.go;drc=6c2841d6cc3eadd23e07912ec331943ee33d7de8;l=161
> >
> > Here is a public link to the line of code in the actual test leading up to the above
> > path (this is the test that is run:
> > network.RoutingFallthrough.ipv4_only_primary) :
> > https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/tast-tests/src/chromiumos/tast/local/bundles/cros/network/routing_fallthrough.go;drc=8fbf2c53960bc8917a6a01fda5405cad7c17201e;l=52
> >
> > > > Using ftrace, I found the callbacks it was queuing which this series fixes. Use
> > > > call_rcu_flush() to revert to the old behavior. With that, the test passes.
> > >
> > > What is this test about ? What barrier was used to make it not flaky ?
> >
> > I provided the links above, let me know if you have any questions.
> >
> > > Was it depending on some undocumented RCU behavior ?
> >
> > This is a new RCU feature posted here for significant power-savings on
> > battery-powered devices:
> > https://lore.kernel.org/rcu/20221017140726.GG5600@paulmck-ThinkPad-P17-Gen-1/T/#m7a54809b8903b41538850194d67eb34f203c752a
> >
> > There is also an LPC presentation about the same, I can dig the link if you
> > are interested.
> >
> > > Maybe adding a sysctl to force the flush would be better for functional tests ?
> > >
> > > I would rather change the test(s), than adding call_rcu_flush(),
> > > adding merge conflicts to future backports.
> >
> > I am not too sure about that, I think a user might expect the network
> > interface to disappear from the networking tables quickly enough without
> > dealing with barriers or kernel iternals. However, I added the authors of the
> > test to this email in the hopes he can provide is point of views as well.
> >
> > The general approach we are taking with this sort of thing is to use
> > call_rcu_flush() which is basically the same as call_rcu() for systems with
> > CALL_RCU_LAZY=n. You can see some examples of that in the patch series link
> > above. Just to note, CALL_RCU_LAZY depends on CONFIG_RCU_NOCB_CPU so its only
> > Android and ChromeOS that are using it. I am adding Jie to share any input,
> > he is from the networking team and knows this test well.
> >
> >
>
> I do not know what is this RCU_LAZY thing, but IMO this should be opt-in

You should read the links I sent you. We did already try opt-in,
Thomas Gleixner made a point at LPC that we should not add new APIs
for this purpose and confuse kernel developers.

> For instance, only kfree_rcu() should use it.

No. Most of the call_rcu() usages are for freeing memory, so the
consensus is we should apply this as opt out and fix issues along the
way. We already did a lot of research/diligence on seeing which users
need conversion.

> We can not review hundreds of call_rcu() call sites and decide if
> adding arbitrary delays cou hurt .

That work has already been done as much as possible, please read the
links I sent.

Thanks.