Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy_rcu

[Joel Fernandes]

On Thu, Nov 17, 2022 at 5:38 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 17, 2022 at 5:17 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 17, 2022 at 7:58 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 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.

Also just to add, this test is a bit weird / corner case, as in anyone
expecting a quick response from call_rcu() is broken by design.
However, for these callbacks, it does not matter much which API they
use as they are quite infrequent for power savings.

Thanks.