Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting

[Johannes Weiner]

On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
> On 4/7/24 12:19 PM, Baolin Wang wrote:
> > On 2024/3/21 02:02, Johannes Weiner wrote:
> >>   
> >> +	account_freepages(page, zone, 1 << order, migratetype);
> >> +
> >>   	while (order < MAX_PAGE_ORDER) {
> >> -		if (compaction_capture(capc, page, order, migratetype)) {
> >> -			__mod_zone_freepage_state(zone, -(1 << order),
> >> -								migratetype);
> >> +		int buddy_mt = migratetype;
> >> +
> >> +		if (compaction_capture(capc, page, order, migratetype))
> >>   			return;
> >> -		}
> > 
> > IIUC, if the released page is captured by compaction, then the 
> > statistics for free pages should be correspondingly decreased, 
> > otherwise, there will be a slight regression for my thpcompact benchmark.
> > 
> > thpcompact Percentage Faults Huge
> >                            k6.9-rc2-base        base + patch10 + 2 fixes	
> > Percentage huge-1        78.18 (   0.00%)       71.92 (  -8.01%)
> > Percentage huge-3        86.70 (   0.00%)       86.07 (  -0.73%)
> > Percentage huge-5        90.26 (   0.00%)       78.02 ( -13.57%)
> > Percentage huge-7        92.34 (   0.00%)       78.67 ( -14.81%)
> > Percentage huge-12       91.18 (   0.00%)       81.04 ( -11.12%)
> > Percentage huge-18       89.00 (   0.00%)       79.57 ( -10.60%)
> > Percentage huge-24       90.52 (   0.00%)       80.07 ( -11.54%)
> > Percentage huge-30       94.44 (   0.00%)       96.28 (   1.95%)
> > Percentage huge-32       93.09 (   0.00%)       99.39 (   6.77%)
> > 
> > I add below fix based on your fix 2, then the thpcompact Percentage 
> > looks good. How do you think for the fix?
> 
> Yeah another well spotted, thanks. "slight regression" is an understatement,
> this affects not just a "statistics" but very important counter
> NR_FREE_PAGES which IIUC would eventually become larger than reality, make
> the watermark checks false positive and result in depleted reserves etc etc.
> Actually wondering why we're not seeing -next failures already (or maybe I
> just haven't noticed).

Good catch indeed.

Trying to understand why I didn't notice this during testing, and I
think it's because I had order-10 pageblocks in my config. There is
this in compaction_capture():

	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
		return false;

Most compaction is for order-9 THPs on movable blocks, so I didn't get
much capturing in practice in order for that leak to be noticable.

In earlier versions of the patches I had more aggressive capturing,
but also did the accounting in add_page_to/del_page_from_freelist(),
so capturing only steals what's already accounted to be off list.

With the __add/__del and the consolidated account_freepages()
optimization, compaction_capture() needs explicit accounting again.

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 8330c5c2de6b..2facf844ef84 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -805,8 +805,10 @@ static inline void __free_one_page(struct page *page,
> >          while (order < MAX_PAGE_ORDER) {
> >                  int buddy_mt = migratetype;
> > 
> > -               if (compaction_capture(capc, page, order, migratetype))
> > +               if (compaction_capture(capc, page, order, migratetype)) {
> > +                       account_freepages(zone, -(1 << order), migratetype);
> >                          return;
> > +               }
> > 
> >                  buddy = find_buddy_page_pfn(page, pfn, order, &buddy_pfn);
> >                  if (!buddy)
> > 
> > With my fix, the THP percentage looks better:
> >                        k6.9-rc2-base          base + patch10 + 2 fixes	+ 
> > my fix
> > Percentage huge-1        78.18 (   0.00%)       82.83 (   5.94%)
> > Percentage huge-3        86.70 (   0.00%)       93.47 (   7.81%)
> > Percentage huge-5        90.26 (   0.00%)       94.73 (   4.95%)
> > Percentage huge-7        92.34 (   0.00%)       95.22 (   3.12%)
> > Percentage huge-12       91.18 (   0.00%)       92.40 (   1.34%)
> > Percentage huge-18       89.00 (   0.00%)       85.39 (  -4.06%)
> > Percentage huge-24       90.52 (   0.00%)       94.70 (   4.61%)
> > Percentage huge-30       94.44 (   0.00%)       97.00 (   2.71%)
> > Percentage huge-32       93.09 (   0.00%)       92.87 (  -0.24%)

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

With fixed indentation:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 70f82635f650..96815e3c22f2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -805,8 +805,10 @@ static inline void __free_one_page(struct page *page,
 	while (order < MAX_PAGE_ORDER) {
 		int buddy_mt = migratetype;
 
-		if (compaction_capture(capc, page, order, migratetype))
+		if (compaction_capture(capc, page, order, migratetype)) {
+			account_freepages(zone, -(1 << order), migratetype);
 			return;
+		}
 
 		buddy = find_buddy_page_pfn(page, pfn, order, &buddy_pfn);
 		if (!buddy)