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. > 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. > > return dst; > } > @@ -1794,6 +1795,7 @@ static void compaction_free(struct folio *dst, unsigned long data) > > list_add(&dst->lru, &cc->freepages); > cc->nr_freepages++; > + cc->nr_migratepages++; > } > > /* possible outcome of isolate_migratepages */
