Re: [PATCH v2 2/2] x86: add prefetching to do_csum

[Joe Perches <joe@perches.com>]

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

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.
 */