On Tue, Jun 01, 2021 at 02:38:03PM +0200, Toke Høiland-Jørgensen wrote: > Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx> writes: > > > Under rare circumstances there might be a situation where a requirement > > of having a XDP Tx queue per core could not be fulfilled and some of the > > Tx resources would have to be shared between cores. This yields a need > > for placing accesses to xdp_rings array onto critical section protected > > by spinlock. > > > > Design of handling such scenario is to at first find out how many queues > > are there that XDP could use. Any number that is not less than the half > > of a count of cores of platform is allowed. XDP queue count < cpu count > > is signalled via new VSI state ICE_VSI_XDP_FALLBACK which carries the > > information further down to Rx rings where new ICE_TX_XDP_LOCKED is set > > based on the mentioned VSI state. This ring flag indicates that locking > > variants for getting/putting xdp_ring need to be used in fast path. > > > > For XDP_REDIRECT the impact on standard case (one XDP ring per CPU) can > > be reduced a bit by providing a separate ndo_xdp_xmit and swap it at > > configuration time. However, due to the fact that net_device_ops struct > > is a const, it is not possible to replace a single ndo, so for the > > locking variant of ndo_xdp_xmit, whole net_device_ops needs to be > > replayed. > > > > It has an impact on performance (1-2 %) of a non-fallback path as > > branches are introduced. > > I generally feel this is the right approach, although the performance > impact is a bit unfortunately, obviously. Maybe it could be avoided by > the use of static_branch? I.e., keep a global refcount of how many > netdevs are using the locked path and only activate the check in the > fast path while that refcount is >0? This would be an ideal solution if we would be able to have it PF-scoped, which AFAICT is not possible as static key is per module, right? I checked that before the bank holiday here in Poland and indeed I was not observing perf drops. Only thing that is questionable is the fact that a single PF would affect all the others that ice driver is serving. OTOH I see that Jesper acked that work. Let me play with this a bit more as I'm in the middle of switching my HW lab, but I wanted to break the silence over here. I didn't manage to check that one fallback path will affect other PFs. Thanks Toke for that great idea :) any other opinions are more than welcome. > > -Toke >
