Re: [PATCH] powerpc/vdso64: inline __get_datapage()

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Wed, Aug 21, 2019 at 02:59:59PM +0530, Santosh Sivaraj wrote:
> except for a couple of calls (1 or 2 nsec reduction), there are no
> improvements in the call times. Or is 10 nsec the minimum granularity??
> 
> So I don't know if its even worth updating vdso64 except to keep vdso32 and
> vdso64 equal.

Calls are cheap, in principle...  It is the LR stuff that can make it
slower on some cores, and a lot of calling sequence stuff may have
considerable overhead of course.

> +.macro get_datapage ptr, tmp
> +	bcl	20,31,888f
> +888:
> +	mflr	\ptr
> +	addi	\ptr, \ptr, __kernel_datapage_offset - 888b
> +	lwz	\tmp, 0(\ptr)
> +	add	\ptr, \tmp, \ptr
> +.endm

(You can just write that as
	bcl 20,31,$+4
	mflr \ptr
etc.  Useless labels are useless :-) )

One thing you might want to do to improve performance is to do this without
the bcl etc., because you cannot really hide the LR latency of that.  But
that isn't very many ns either...  Superscalar helps, OoO helps, but it is
mostly just that >100MHz helps ;-)


Segher