Re: [PATCH] mm/page_alloc: remove prefetchw() on freeing page to buddy system

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jul 02, 2024 at 08:57:57AM +0200, David Hildenbrand wrote:
> > I did 10 round bootup tests before and after this change, the data
> > doesn't prove prefetchw() help speeding up bootmem freeing. The sum of
> > the 10 round bootmem freeing time after prefetchw() removal even 5.2%
> > faster than before.
> 
> I suspect this is noise, though.

I think it's real, though small.  Each prefetchw() is an instruction,
and if we can avoid issuing an instruction, we should.

> Something like:
> 
> for (;;) {
> 	...
> 	if (++loop >= nr_pages)
> 		break;
> 	p++;
> }
> 
> 
> Might generate slightly better code, because we know that we execute the
> loop body at least once. We use that in set_ptes(), for example.

I don't think it's worth doing.  Keep the loop simple and obvious.
set_ptes() is different because we actually expect to execute the loop
exactly once (ie most folios are small).  So two compares per call to
set_ptes() instead of one makes a difference.  Here, we're expecting
to execute this loop, what, a million times?  Doing a million-and-one
compares instead of a million makes no observable difference.

I would like to see v2 of this patch dropped, please Andrew.