linux-kernel - Re: [PATCH v2 2/2] x86: add prefetching to do

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Message-ID: <20131106200204.GB11415@hmsreliant.think-freely.org>
Date:	Wed, 6 Nov 2013 15:02:04 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	sebastien.dugue@...l.net, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum

On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
> On Wed, 2013-11-06 at 10:54 -0500, Neil Horman wrote:
> > On Wed, Nov 06, 2013 at 10:34:29AM -0500, Dave Jones wrote:
> > > On Wed, Nov 06, 2013 at 10:23:19AM -0500, Neil Horman wrote:
> > >  > do_csum was identified via perf recently as a hot spot when doing
> > >  > receive on ip over infiniband workloads.  After alot of testing and
> > >  > ideas, we found the best optimization available to us currently is to
> > >  > prefetch the entire data buffer prior to doing the checksum
> []
> > I'll fix this up and send a v3, but I'll give it a day in case there are more
> > comments first.
> 
> Perhaps a reduction in prefetch loop count helps.
> 
> Was capping the amount prefetched and letting the
> hardware prefetch also tested?
> 
> 	prefetch_lines(buff, min(len, cache_line_size() * 8u));
> 
> Also pedantry/trivial comments:
> 
> __always_inline instead of inline
> static __always_inline void prefetch_lines(const void *addr, size_t len)
> {
> 	const void *end = addr + len;
> ...
> 
> buff doesn't need a void * cast in prefetch_lines
> 
Actually I take back what I said here, we do need the cast, not for a conversion
from unsigned char * to void *, but rather to discard the const qualifier
without making the compiler complain.

Neil

> Beside the commit message, the comment above prefetch_lines
> also needs updating to remove the "Manual Prefetching" line.
> 
> /*
>  * Do a 64-bit checksum on an arbitrary memory area.
>  * Returns a 32bit checksum.
>  *
>  * This isn't as time critical as it used to be because many NICs
>  * do hardware checksumming these days.
>  * 
>  * Things tried and found to not make it faster:
>  * Manual Prefetching
>  * Unrolling to an 128 bytes inner loop.
>  */
> 
> 
> 
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