On Tue, May 30, 2017 at 11:56:30AM -0400, Vivien Didelot wrote: > Hi Andrew, David, > > David Miller <da...@davemloft.net> writes: > > >>> +bool dsa_uses_tagged_protocol(struct dsa_switch_tree *dst) > >>> +{ > >>> + return !!dst->rcv; > >>> +} > >>> + > >> > >> You need to be careful here. This is in the hot path. Every frame > >> received uses this code. And think about a distro kernel, which might > >> have DSA enabled by default, yet is unlikely to have any switches. You > >> are adding a function call which can be called millions of times per > >> second.... > > > > Yeah, we really can't make this change. > > > > This isn't glibc where we're trying to hide the implementation of "FILE *" > > behind accessor functions that caller can't see. We inline things when > > performance dictates, and it does here. > > Thanks for the explanation, this wasn't obvious to me at all. So inline > is mandatory here. Would a dereference like "!!dst->tag_ops->rcv" have > an significant impact on performance?
The additional dereference could cause a cache miss when accessing tag_ops, which is expensive. dst will be in cache, so dst->rcv should always be cheap. Andrew