> On Nov 17, 2022, at 2:29 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Thu, Nov 17, 2022 at 05:40:40PM +0000, Joel Fernandes wrote: >>> 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. > > The "broken by design" is a bit strong. Some of those call_rcu() > invocations have been around for the better part of 20 years, after all. > > That aside, I do hope that we can arrive at something that will enhance > battery lifetime while avoiding unnecessary disruption. But we are > unlikely to be able to completely avoid disruption. As this email > thread illustrates. ;-) Another approach, with these 3 patches could be to keep the call_rcu() but add an rcu_barrier() after them. I think people running ip del netns should not have to wait for their RCU cb to take too long to run and remove user visible state. But I would need suggestions from networking experts which CBs of these 3, to do this for. Or for all of them. Alternatively, we can also patch just the test with a new knob that does rcu_barrier. But I dislike that as it does not fix it for all users. Probably the ip utilities will also need a patch then. Thanks, - Joel > > Thanx, Paul
