Re: [PATCH net-next] openvswitch: enable ops lock for internal ports

[Wang Zhan]

> Why are you sending this patch? Do you have an actual real life need
> for unlocked get_drvinfo??

Yes. The use case is broader than unlocked get_drvinfo.

We are trying to reduce cases where ovs-vswitchd gets blocked waiting for
the global RTNL lock. In production we saw ovs-vswitchd repeatedly report
"Unreasonably long" poll intervals. Tracing showed one OVS operation waiting
about 500 ms for RTNL while node-exporter was collecting the speed of an
mlx5 interface. The node-exporter process was CPU quota throttled, which
stretched that RTNL hold time; mlx5 can make this easier to hit because some
queries go through firmware/mailbox paths.

netdev_linux_set_ol() is one OVS path that can take RTNL. With
userspace-tso-enable, OVS uses this path to read ethtool feature information
through ETHTOOL_GSSET_INFO, ETHTOOL_GSTRINGS and ETHTOOL_GFEATURES. For
ovs-internal-port, these GET operations do not need RTNL, but currently take
it because internal ports do not opt into per-device ops locking.