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 15:35:24 +0200
From:	Christophe Leroy <christophe.leroy@....fr>
To:	Scott Wood <scottwood@...escale.com>
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



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.

Christophe

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