[PATCH] powerpc/vdso64: inline __get_datapage()

[Christophe Leroy]

Le 21/08/2019 à 13:44, Segher Boessenkool a écrit :
> 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.

On an 8xx, a taken branch is 2 cycles and a non taken branch in 1 cycle 
(+ the refetch if that was not the anticipate branch).

> 
>> +.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 :-) )

Nice trick. Will use that.

> 
> 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 ;-)

Good idea. Did you have a look at my vdso32 similar patch ? 
https://patchwork.ozlabs.org/patch/1148274/

Do you have any idea on how to avoid that bcl/mflr stuff ?

Christophe