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Date:   Fri, 15 Jul 2022 15:13:13 +0100
From:   William Lam <william.lam@bytedance.com>
To:     Punit Agrawal <punit.agrawal@bytedance.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: compaction: include compound page count for scanning
 in pageblock isolation
Message-ID: <YtF1+bv0gPB7pqNL@C02FT09GML7L>
References: <20220711202806.22296-1-william.lam@bytedance.com>
 <874jzin30k.fsf@stealth>
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On Fri, Jul 15, 2022 at 10:23:07AM +0100, Punit Agrawal wrote:
> Hi William,
> 
> William Lam <william.lam@bytedance.com> writes:
> 
> > The number of scanned pages can be lower than the number of isolated
> > pages when isolating mirgratable or free pageblock. The metric is being
> > reported in trace event and also used in vmstat.
> >
> > This behaviour is confusing since currently the count for isolated pages
> > takes account of compound page but not for the case of scanned pages.
> > And given that the number of isolated pages(nr_taken) reported in
> > mm_compaction_isolate_template trace event is on a single-page basis,
> > the ambiguity when reporting the number of scanned pages can be removed
> > by also including compound page count.
> 
> A minor suggestion - It maybe useful to include an example trace output
> to highlight the issue.
> 

some example output from trace where it shows nr_taken can be greater
than nr_scanned:

Produced by kernel v5.19-rc6
kcompactd0-42      [001] .....  1210.268022: mm_compaction_isolate_migratepages: range=(0x107ae4 ~ 0x107c00) nr_scanned=265 nr_taken=255
[...]
kcompactd0-42      [001] .....  1210.268382: mm_compaction_isolate_freepages: range=(0x215800 ~ 0x215a00) nr_scanned=13 nr_taken=128
kcompactd0-42      [001] .....  1210.268383: mm_compaction_isolate_freepages: range=(0x215600 ~ 0x215680) nr_scanned=1 nr_taken=128

mm_compaction_isolate_migratepages does not seem to have this behaviour,
but for the reason of consistency, nr_scanned should also be taken care
of in that side.

> >
> > Signed-off-by: William Lam <william.lam@bytedance.com>
> > ---
> >  mm/compaction.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 1f89b969c12b..1b51cf2d32b6 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -616,6 +616,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> >  			break;
> >  		set_page_private(page, order);
> >  
> > +		nr_scanned += isolated - 1;
> >  		total_isolated += isolated;
> >  		cc->nr_freepages += isolated;
> >  		list_add_tail(&page->lru, freelist);
> > @@ -1101,6 +1102,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >  isolate_success_no_list:
> >  		cc->nr_migratepages += compound_nr(page);
> >  		nr_isolated += compound_nr(page);
> > +		nr_scanned += compound_nr(page) - 1;
> >  
> >  		/*
> >  		 * Avoid isolating too much unless this block is being
> > @@ -1504,6 +1506,7 @@ fast_isolate_freepages(struct compact_control *cc)
> >  			if (__isolate_free_page(page, order)) {
> >  				set_page_private(page, order);
> >  				nr_isolated = 1 << order;
> > +				nr_scanned += nr_isolated - 1;
> >  				cc->nr_freepages += nr_isolated;
> >  				list_add_tail(&page->lru, &cc->freepages);
> >  				count_compact_events(COMPACTISOLATED, nr_isolated);
> 
> Regardless of the comment above -
> 
> Reviewed-by: Punit Agrawal <punit.agrawal@bytedance.com>