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

Han Zhou zhouhan at gmail.com
Fri Aug 30 21:17:47 UTC 2019


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,
Han
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