Re: [PATCH 2/2] mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

[Baolin Wang]

On 2024/2/12 18:32, Vlastimil Babka wrote:
> On 1/22/24 14:01, Baolin Wang wrote:
> > Currently we will use 'cc->nr_freepages >= cc->nr_migratepages' comparison
> > to ensure that enough freepages are isolated in isolate_freepages(), however
> > it just decreases the cc->nr_freepages without updating cc->nr_migratepages
> > in compaction_alloc(), which will waste more CPU cycles and cause too many
> > freepages to be isolated.
>
> Hm yeah I guess this can happen with fast_isolate_freepages() if it returns
> with something but not all the freepages that are expected to be needed, and
> then we get to isolate_freepages() again.

Yes.

> > So we should also update the cc->nr_migratepages when allocating or freeing
> > the freepages to avoid isolating excess freepages. And I can see fewer free
> > pages are scanned and isolated when running thpcompact on my Arm64 server:
> >                                         k6.7         k6.7_patched
> > Ops Compaction pages isolated      120692036.00   118160797.00
> > Ops Compaction migrate scanned     131210329.00   154093268.00
> > Ops Compaction free scanned       1090587971.00  1080632536.00
> > Ops Compact scan efficiency               12.03          14.26
> >
> > Moreover, I did not see an obvious latency improvements, this is likely because
> > isolating freepages is not the bottleneck in the thpcompact test case.
> >                                k6.7                  k6.7_patched
> > Amean     fault-both-1      1089.76 (   0.00%)     1080.16 *   0.88%*
> > Amean     fault-both-3      1616.48 (   0.00%)     1636.65 *  -1.25%*
> > Amean     fault-both-5      2266.66 (   0.00%)     2219.20 *   2.09%*
> > Amean     fault-both-7      2909.84 (   0.00%)     2801.90 *   3.71%*
> > Amean     fault-both-12     4861.26 (   0.00%)     4733.25 *   2.63%*
> > Amean     fault-both-18     7351.11 (   0.00%)     6950.51 *   5.45%*
> > Amean     fault-both-24     9059.30 (   0.00%)     9159.99 *  -1.11%*
> > Amean     fault-both-30    10685.68 (   0.00%)    11399.02 *  -6.68%*
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/compaction.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 066b72b3471a..6c84e3a5b32b 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1779,6 +1779,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
> >   	dst = list_entry(cc->freepages.next, struct folio, lru);
> >   	list_del(&dst->lru);
> >   	cc->nr_freepages--;
> > +	cc->nr_migratepages--;
>
> This is breaking the tracepoint TRACE_EVENT(mm_compaction_migratepages)
> which does
>
> __entry->nr_failed = cc->nr_migratepages - nr_succeeded;
>
> and is called after migrate_pages() finishes, so now this will underflow.
>
> Probably need to get a snapshot of cc->nr_migratepages before calling
> migrate_pages() and then feed that to the tracepoint instead of cc.

Ah, good catch. Will fix in next version. Thanks.