Ilya Maximets <i.maxim...@ovn.org> writes: > On 9/29/23 09:06, Nicholas Piggin wrote: >> On Wed Sep 27, 2023 at 6:36 PM AEST, Ilya Maximets wrote: >>> On 9/27/23 02:13, Nicholas Piggin wrote: >>>> Hi, >>>> >>>> We've got a report of a stack overflow on ppc64le with a 16kB kernel >>>> stack. Openvswitch is just one of many things in the stack, but it >>>> does cause recursion and contributes to some usage. >>>> >>>> Here are a few patches for reducing stack overhead. I don't know the >>>> code well so consider them just ideas. GFP_ATOMIC allocations >>>> introduced in a couple of places might be controversial, but there >>>> is still some savings to be had if you skip those. >>>> >>>> Here is one place detected where the stack reaches >14kB before >>>> overflowing a little later. I massaged the output so it just shows >>>> the stack frame address on the left. >>> >>> Hi, Nicholas. Thanks for the patches! >>> >>> Though it looks like OVS is not really playing a huge role in the >>> stack trace below. How much of the stack does the patch set save >>> in total? How much patches 2-7 contribute (I posted a patch similar >>> to the first one last week, so we may not count it)? >> >> Stack usage was tested for the same path (this is backported to >> RHEL9 kernel), and saving was 2080 bytes for that. It's enough >> to get us out of trouble. But if it was a config that caused more >> recursions then it might still be a problem. > > The 2K total value likely means that only patches 1 and 4 actually > contribute much into the savings. And I agree that running at > 85%+ stack utilization seems risky. It can likely be overflowed > by just a few more recirculations in OVS pipeline or traversing > one more network namespace on a way out. And it's possible that > some of the traffic will take such a route in your system even if > you didn't see it yet. > >>> Also, most of the changes introduced here has a real chance to >>> noticeably impact performance. Did you run any performance tests >>> with this to assess the impact? >> >> Some numbers were posted by Aaron as you would see. 2-4% for that >> patch, but I suspect the rest should have much smaller impact. > > They also seem to have a very small impact on the stack usage, > so may be not worth touching at all, since performance evaluation > for them will be necessary before they can be accepted.
Actually, it's also important to keep in mind that the vport_receive is only happening once in my performance test. I expect it gets worse when running in the scenario (br-ex, br-int, br-tun setup). >> >> Maybe patch 2 if you were doing a lot of push_nsh operations, but >> that might be less important since it's out of the recursive path. > > It's also unlikely that you have NHS pipeline configured in OVS. > >> >>> >>> One last thing is that at least some of the patches seem to change >>> non-inlined non-recursive functions. Seems unnecessary. >>> >>> Best regards, Ilya Maximets. >>> >> >> One thing I do notice in the trace: >> >> >> clone_execute is an action which can be deferred AFAIKS, but it is >> not deferred until several recursions deep. >> >> If we deferred always when possible, then might avoid such a big >> stack (at least for this config). Is it very costly to defer? Would >> it help here, or is it just going to process it right away and >> cause basically the same call chain? > > It may save at most two stack frames maybe, because deferred actions > will be called just one function above in ovs_execute_actions(), and > it will not save us from packets exiting openvswitch module and > re-entering from a different port, which is a case in the provided > trace. It used to always be deferred but, IIUC we were hitting deferred actions limit quite a bit so when the sample and clone actions were unified there was a choice to recurse to avoid dropping packets. > Also, I'd vote against deferring, because then we'll start hitting > the limit of deferred actions much faster causing packet drops, which > is already a problem for some OVN deployments. And deferring involves > copying a lot of memory, which will hit performance once again. > > Best regards, Ilya Maximets. _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev
