Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged_protocol code

[Andrew Lunn <andrew@lunn.ch>]

On Tue, May 30, 2017 at 11:56:30AM -0400, Vivien Didelot wrote:
> Hi Andrew, David,
> 
> David Miller <davem@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