Hi Andrew, Andrew Lunn <and...@lunn.ch> writes:
> 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. OK! That was interesting. I'm dropping the first 2 patches. Thanks, Vivien