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

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Date:	Tue, 6 Oct 2015 11:46:35 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Christophe Leroy <christophe.leroy@....fr>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 01/25] powerpc/8xx: Save r3 all the time in DTLB miss
 handler

On Tue, 2015-10-06 at 15:35 +0200, Christophe Leroy wrote:
> Le 29/09/2015 00:07, Scott Wood a écrit :
> > 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@....fr>
> > 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.

FWIW, the added instruction is an SPR access and I doubt that's only one 
cycle.

-Scott


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