Re: [PATCH] mm: page_alloc: remove branch operation in free_pages_prepare()

[Will Huck]

Hi Hugh,
On 03/08/2013 10:01 AM, Hugh Dickins wrote:
> On Fri, 8 Mar 2013, Joonsoo Kim wrote:
> > On Thu, Mar 07, 2013 at 10:54:15AM -0800, Hugh Dickins wrote:
> > > On Thu, 7 Mar 2013, Joonsoo Kim wrote:
> > >
> > > > When we found that the flag has a bit of PAGE_FLAGS_CHECK_AT_PREP,
> > > > we reset the flag. If we always reset the flag, we can reduce one
> > > > branch operation. So remove it.
> > > >
> > > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > I don't object to this patch.  But certainly I would have written it
> > > that way in order not to dirty a cacheline unnecessarily.  It may be
> > > obvious to you that the cacheline in question is almost always already
> > > dirty, and the branch almost always more expensive.  But I'll leave that
> > > to you, and to those who know more about these subtle costs than I do.
> > Yes. I already think about that. I thought that even if a cacheline is
> > not dirty at this time, we always touch the 'struct page' in
> > set_freepage_migratetype() a little later, so dirtying is not the problem.
> I expect that a very high proportion of user pages have
> PG_uptodate to be cleared here; and there's also the recently added

When PG_uptodate will be set?

> page_nid_reset_last(), which will dirty the flags or a nearby field
> when CONFIG_NUMA_BALANCING.  Those argue in favour of your patch.
>
> > But, now, I re-think this and decide to drop this patch.
> > The reason is that 'struct page' of 'compound pages' may not be dirty
> > at this time and will not be dirty at later time.
> Actual compound pages would have PG_head or PG_tail or PG_compound
> to be cleared there, I believe (check if I'm right on that).  The
> questionable case is the ordinary order>0 case without __GFP_COMP
> (and page_nid_reset_last() is applied to each subpage of those).
>
> > So this patch is bad idea.
> I'm not so sure.  I doubt your patch will make a giant improvement
> in kernel performance!  But it might make a little - maybe you just
> need to give some numbers from perf to justify it (but I'm easily
> dazzled by numbers - don't expect me to judge the result).
>
> Hugh
>
> > Is there any comments?
> >
> > Thanks.
> >
> > > Hugh
> > >
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index 8fcced7..778f2a9 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -614,8 +614,7 @@ static inline int free_pages_check(struct page *page)
> > > >   		return 1;
> > > >   	}
> > > >   	page_nid_reset_last(page);
> > > > -	if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
> > > > -		page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> > > > +	page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> > > >   	return 0;
> > > >   }
> > > >   
> > > --
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