coredns should not be running on master by default

[bhegazy]

CoreDNS can be scheduled on masters.

The masters are busy enough and coredns shouldn't be running there as well

I have followed coredns/deployment#50

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[rdrgmnzs]

@bhegazy we tend to lean more towards the configs used by kubeadm. Also at this point we would have to be very careful about this change so that we don't break configs for anyone that may be relying on coreDNS being deployed on the masters, so we most likely don't want to just outright remove it.

At this point if someone does not want CoreDNS deployed to their masters we'd have to make this configurable and most likely leave the default such that it is still deployed to the masters.

[bhegazy]

@rdrgmnzs You're absolutely right, I have requested this change due to my m5.xlarge masters was always above 70% cpu because of CoreDNS pods running ( we have around 50k incoming requests to CoreDNS coredns_dns_request_count_total). after moving to worker nodes only cpu now 35% so I would really love to help and make it configurable and leave this default, if you can guide me as I'm new here 👍

[justinsb]

So it's not actually the kubeadm configs, but the GCE kube-up configs - they are the ones that are very heavily tested. The file seems to be here: https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/coredns/coredns.yaml.in

But ... I actually agree with you - there's no reason to run CoreDNS on the master, and we didn't run kube-dns on the master. I'm going to open an issue upstream with kubernetes to see what the logic is.

[justinsb]

Raised upstream in kubernetes/kubernetes#70851 - it will be good to see what they say. This does seem like a change from the kube-dns behaviour.

[justinsb]

Upstream seemed to agree, and I think it will be very surprising to people if this behaves differently to kube-dns.

Thanks @bhegazy

/approve
/lgtm
/ok-to-test

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[bhegazy]

This is awesome Thanks @justinsb