Re: [PATCH 1/2] IB/isert: use unlikely macro in the fast path

[Leon Romanovsky <leonro@mellanox.com>]

On Thu, Aug 06, 2020 at 12:51:15PM -0700, Sagi Grimberg wrote:
>
> > I reviewed this patch and didn't find any justification for performance
> > claim, can you please provide us numbers before/after so we will be able
> > to decide based on reliable data? It will help us to review our drivers
> > and improve them even more.
>
> I don't see any reason to find evidence in justification here. It's a
> fastpath call, which is unlikely to fail, and these macros are
> considered common practice.
>
> There is no reason to make Max to go and quantify a micro-optimization.

Unfortunately Max didn't try to see if these likely/unlikely macros
change something, but I did.

Simple objdump -d before and after shows that GCC 9 generates same
ISERT code before and after this patch. It is expected and there are a lot
of reasons for that, but all of them can be reduced to two:
* First, GCC is awesome in building profiled code with right predictions for
standard flows.
* Second, likely/unlikely is intended to be used when input/output is random
from GCC point of view.

So as a summary, there is no optimization here, just misuse of unlikely macro.

BTW, old GCCs behave the same and kernel full of wrong copy/paste.

Thanks