On Fri, Jul 15, 2022 at 10:23:07AM +0100, Punit Agrawal wrote:
> Hi William,
>
> William Lam <william.lam@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxx>
