Re: [PATCH v2 01/25] powerpc/8xx: Save r3 all the time in DTLB miss handler

[Scott Wood]

On Tue, 2015-10-06 at 15:35 +0200, Christophe Leroy wrote:
> Le 29/09/2015 00:07, Scott Wood a Ãcrit :
> > On Tue, Sep 22, 2015 at 06:50:29PM +0200, Christophe Leroy wrote:
> > > We are spending between 40 and 160 cycles with a mean of 65 cycles in
> > > the TLB handling routines (measured with mftbl) so make it more
> > > simple althought it adds one instruction.
> > > 
> > > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > Does this just make it simpler or does it make it faster?  What is the
> > performance impact?  Is the performance impact seen with or without
> > CONFIG_8xx_CPU6 enabled?  Without it, it looks like you're adding an
> > mtspr/mfspr combo in order to replace one mfspr.
> > 
> > 
> The performance impact is not noticeable. Theoritically it adds 1 cycle 
> on a mean of 65 cycles, that is 1.5%. Even in the worst case where we 
> spend around 10% of the time in TLB handling exceptions, that represents 
> only 0.15% of the total CPU time. So that's almost nothing.
> Behind the fact to get in simpler, the main reason is because I need a 
> third register for the following patch in the set, otherwise I would 
> spend a more time saving and restoring CR several times.

If you had said in the changelog that it was because future patches would 
need the register to be saved, we could have avoided this exchange...  
Especially with large patchsets, I review the patches one at a time.  Don't 
assume I know what's coming in patch n+1 (and especially not n+m) when I 
review patch n.

-Scott

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