linux-kernel - Re: [PATCH] ARM64: Improve copy_page for 128 cache line sizes.

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Message-Id: <201512211442.58803.arnd@arndb.de>
Date:	Mon, 21 Dec 2015 14:42:58 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Will Deacon <will.deacon@....com>,
	Andrew Pinski <apinski@...ium.com>, pinsia@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM64: Improve copy_page for 128 cache line sizes.

On Monday 21 December 2015, Will Deacon wrote:
> On Sat, Dec 19, 2015 at 04:11:18PM -0800, Andrew Pinski wrote:
> > Adding a check for the cache line size is not much overhead.
> > Special case 128 byte cache line size.
> > This improves copy_page by 85% on ThunderX compared to the
> > original implementation.
> 
> So this patch seems to:
> 
>   - Align the loop
>   - Increase the prefetch size
>   - Unroll the loop once
> 
> Do you know where your 85% boost comes from between these? I'd really
> like to avoid having multiple versions of copy_page, if possible, but
> maybe we could end up with something that works well enough regardless
> of cacheline size. Understanding what your bottleneck is would help to
> lead us in the right direction.
> 
> Also, how are you measuring the improvement? If you can share your
> test somewhere, I can see how it affects the other systems I have access
> to.

A related question would be how other CPU cores are affected by the change.
The test for the cache line size is going to take a few cycles, possibly
a lot on certain implementations, e.g. if we ever get one where 'mrs' is
microcoded or trapped by a hypervisor.

Are there any possible downsides to using the ThunderX version on other
microarchitectures too and skip the check?

	Arnd
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