18/04/2019 19:09, Adrien Mazarguil: > On Thu, Apr 18, 2019 at 06:54:22PM +0200, Thomas Monjalon wrote: > <snip> > > > <snip> > > > > > "slave" is a wording from bonding. > > > > > In failsafe, it is sub-device, isn't it? > > > > > > I don't mind, although grep shows a couple of comments talking about > > > slaves > > > already. Either way I think it fits as those are failsafe's pets, as in > > > failsafe does whatever it wants to them and they don't have a say :) > > > > > > Does it warrant a v3? > > > > Yes please, except if Ferruh is already doing the change on apply. > > Will do. > > <snip> > > > > > I'm afraid the performance drop to be hard. > > > > > > Mbufs are still hot from the oven at this stage, so it's not *that* > > > expensive. I don't see a more efficient approach. > > > > Yes, Ali did some quick tests showing no perf drop. > > Great. > > > > > > How the port id in mbuf is used exactly? > > > > > > Applications that dissociate Rx itself from packet processing, or > > > whenever a > > > networking stack is involved. Basically every time some code wonders > > > where a > > > packet comes from due to lack of context and looks at m->port for the > > > answer (e.g. checking that a packet arrives on the right port given its > > > destination address). > > > > > > > > What crash are you seeing? > > > > > > None, thankfully. In my specific use case, 6WINDGate's stack simply drops > > > traffic coming from unknown ports. > > > > > > However nothing prevents applications from using m->port as an index of > > > some > > > array they allocated to quickly retrieve port context without looking it > > > up. They wouldn't expect indices they do not know about in there; assuming > > > it will result in a crash is not far fetched. > > > > > > > Another way to fix it without performance drop would be to add > > > > a new driver op to set the top-level port id. > > > > This top-level id would be stored in the private structure of the port, > > > > initialized with the port id of the port itself, and used to fill mbufs. > > > > > > > > Thoughts? > > > > > > Adding a new devop as a fix would be a problem for stable releases, so > > > this > > > patch is definitely needed, at least as a first step. > > > > > > I'm not against a new API, however would it be worth the trouble? > > > Especially > > > considering it would only be used by failsafe-like drivers with something > > > to > > > hide from applications which is not the main use case. > > > > > > For some PMDs, this operation could only be done at init time before port > > > ID > > > is stored in private Rx queue data for fast retrieval. Retrieving it > > > through > > > a pointer so it can be updated anytime would make it more expensive than > > > necessary for them. > > > > I don't understand this comment. > > The port id is currently retrieved via some pointers already. > > I suggest to look at private structure, it is not different. > > See "rep->port = rxq->port_id" in mlx4_rxtx.c for instance. Port ID is > cached in private queue data structure (struct rxq) and retrieved there to > avoid looking it up in non-local data structure rxq->priv->dev_data->port. > In fact rxq->priv is not accessed even once during Rx.
OK, thanks for the explanation. > > > It's understood that having failsafe in the dataplane has a cost, but even > > > with the proposed fix, that cost is dwarfed by the amount of work done by > > > a > > > true PMD (and the application) for Rx processing. > > > > > > My suggestion is to wait for someone to complain about the performance > > > compared to what they had before that fix, only then see what we can do. > > > > OK
