[ovs-discuss] ovn-controller is taking 100% CPU all the time in one deployment

Numan Siddique nusiddiq at redhat.com
Mon Sep 2 16:35:16 UTC 2019


On Sat, Aug 31, 2019 at 2:48 AM Han Zhou <zhouhan at gmail.com> wrote:

>
>
> On Thu, Aug 29, 2019 at 11:36 PM Numan Siddique <nusiddiq at redhat.com>
> wrote:
> >
> >
> >
> > On Fri, Aug 30, 2019 at 1:04 AM Han Zhou <zhouhan at gmail.com> wrote:
> >>
> >>
> >>
> >> On Thu, Aug 29, 2019 at 12:16 PM Numan Siddique <nusiddiq at redhat.com>
> wrote:
> >>>
> >>>
> >>>
> >>> On Fri, Aug 30, 2019 at 12:37 AM Han Zhou <zhouhan at gmail.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Thu, Aug 29, 2019 at 11:40 AM Numan Siddique <nusiddiq at redhat.com>
> wrote:
> >>>> >
> >>>> > Hello Everyone,
> >>>> >
> >>>> > In one of the OVN deployments, we are seeing 100% CPU usage by
> ovn-controllers all the time.
> >>>> >
> >>>> > After investigations we found the below
> >>>> >
> >>>> >  - ovn-controller is taking more than 20 seconds to complete full
> loop (mainly in lflow_run() function)
> >>>> >
> >>>> >  - The physical switch is sending GARPs periodically every 10
> seconds.
> >>>> >
> >>>> >  - There is ovn-bridge-mappings configured and these GARP packets
> reaches br-int via the patch port.
> >>>> >
> >>>> >  - We have a flow in router pipeline which applies the action -
> put_arp
> >>>> > if it is arp packet.
> >>>> >
> >>>> >  - ovn-controller pinctrl thread receives these garps, stores the
> learnt mac-ips in the 'put_mac_bindings' hmap and notifies the
> ovn-controller main thread by incrementing the seq no.
> >>>> >
> >>>> >  - In the ovn-controller main thread, after lflow_run() finishes,
> pinctrl_wait() is called. This function calls - poll_immediate_wake() as
> 'put_mac_bindings' hmap is not empty.
> >>>> >
> >>>> > - This causes the ovn-controller poll_block() to not sleep at all
> and this repeats all the time resulting in 100% cpu usage.
> >>>> >
> >>>> > The deployment has OVS/OVN 2.9.  We have back ported the
> pinctrl_thread patch.
> >>>> >
> >>>> > Some time back I had reported an issue about lflow_run() taking lot
> of time -
> https://mail.openvswitch.org/pipermail/ovs-dev/2019-July/360414.html
> >>>> >
> >>>> > I think we need to improve the logical processing sooner or later.
> >>>> >
> >>>> > But to fix this issue urgently, we are thinking of the below
> approach.
> >>>> >
> >>>> >  - pinctrl_thread will locally cache the mac_binding entries (just
> like it caches the dns entries). (Please note pinctrl_thread can not access
> the SB DB IDL).
> >>>> >
> >>>> > - Upon receiving any arp packet (via the put_arp action),
> pinctrl_thread will check the local mac_binding cache and will only wake up
> the main ovn-controller thread only if the mac_binding update is required.
> >>>> >
> >>>> > This approach will solve the issue since the MAC sent by the
> physical switches will not change. So there is no need to wake up
> ovn-controller main thread.
> >>>> >
> >>>> > In the present master/2.12 these GARPs will not cause this 100% cpu
> loop issue because incremental processing will not recompute flows.
> >>>> >
> >>>> > Even though the above approach is not really required for
> master/2.12, I think it is still Ok to have this as there is no harm.
> >>>> >
> >>>> > I would like to know your comments and any concerns if any.
> >>>> >
> >>>> > Thanks
> >>>> > Numan
> >>>> >
> >>>>
> >>>> Hi Numan,
> >>>>
> >>>> I think this approach should work. Just to make sure, to update the
> cache efficiently (to avoid another kind of recompute), it should use ovsdb
> change-tracking to update it incrementally.
> >>>>
> >>>> Regarding master/2.12, it is not harmful except that it will add some
> more code and increase memory footprint. For our current use cases, there
> can be easily 10,000s mac_bindings, but it may still be ok because each
> entry is very small. However, is there any benefit for doing this in
> master/2.12?
> >>>
> >>>
> >>> I don't see much benefit. But I can't submit a patch to branch 2.9
> without the fix getting merged in master first right ?
> >>> May be once it is merged in branch 2.9, we can consider to delete it ?
> >>>
> >> I think it is just about how would you maintain a downstream branch.
> Since it is downstream, I don't think you need a change to be in upstream
> before fixing a problem. In this case it may be *no harm*, but what if the
> upstream is completely changed and incompatible for such a fix any more? It
> shouldn't prevent you from fixing your downstream. (Of course it is better
> to not have downstream at all, but sometimes it is useful to have it for a
> temporary period, and since you (and us, too) are already there ... :)
> >
> >
> > The dowstream 2.9 what we have is - OVS 2.9.0 + a bunch of patches (to
> fix issues) which are already merged upstream (preferably upstream branch
> or at least upstream master).  Any downstream only patch is frowned upon.
> When we updrade to 2.10 or higher versions there is  a risk of functional
> changes if the patch is not upstream.
> >
> > If we have apply the approach I described above to downstream 2.9 then
> there is definitely some functional change. When such GARPs are received,
> in the case of our downstream 2.9 we will not wake up ovn-controller main
> thread
> > but with 2.12/master, we wake up the ovn-controller main thread.
> >
> > I still see no harm in having this in upstream master. May be instead of
> having a complete clone of mac_bindings, we can have a subset of
> mac_bindings cached only if those mac_bindings are learnt by an
> ovn-controller.
> >
> > I will explore more.
> >
> > Thanks
> > Numan
> >
>
> Hi Numan,
>
> Your concern is valid, but it just does not make sense to change upstream
> only for old branch (or downstream) maintenance purposes. We should
> consider changing the upstream if there is a benefit.
>
> If there is a problem in downstream that is already solved in upstream,
> there are 3 options:
> 1. upgrade to upstream
> 2. backport the upstream solution to downstream
> 3. fix the downstream with a solution that is different from upstream
>
> In this case I think 3) is what suits you well because 1) is risky because
> it is just released, and 2) might need too much effort.
> In fact 2) may also be a good option in this case. You only need the first
> 3 patches of I-P to avoid the recomputing triggered by pinctrl (up to this
> commit: ovn-controller: Initial use of incremental engine - quiet mode.)
>

Thanks for these suggestions. Suggestion (2) seems a good option.
I tried backporting the first 3 I-P patches to 2.9. But I couldn't. OVS 2.9
doesn't have the indexing patch [1]
which makes backporting these patches really hard.

I think back porting to 2.11 seems doable to me. I will try it out with our
downstream 2.11.

Thanks
Numan


>
> Thanks,
> Han
>
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