mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

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.

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%*

Link: https://lkml.kernel.org/r/644[e-mail]
Signed-off-by: Baolin Wang <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Reviewed-by: Vlastimil Babka <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/trace/events/compaction.h

···
 #ifdef CONFIG_COMPACTION
 TRACE_EVENT(mm_compaction_migratepages,
 
-	TP_PROTO(struct compact_control *cc,
+	TP_PROTO(unsigned int nr_migratepages,
 		unsigned int nr_succeeded),
 
-	TP_ARGS(cc, nr_succeeded),
+	TP_ARGS(nr_migratepages, nr_succeeded),
 
 	TP_STRUCT__entry(
 		__field(unsigned long, nr_migrated)
···
 
 	TP_fast_assign(
 		__entry->nr_migrated = nr_succeeded;
-		__entry->nr_failed = cc->nr_migratepages - nr_succeeded;
+		__entry->nr_failed = nr_migratepages - nr_succeeded;
 	),
 
 	TP_printk("nr_migrated=%lu nr_failed=%lu",

mm/compaction.c

···
 	dst = list_entry(cc->freepages.next, struct folio, lru);
 	list_del(&dst->lru);
 	cc->nr_freepages--;
+	cc->nr_migratepages--;
 
 	return dst;
 }
···
 
 	list_add(&dst->lru, &cc->freepages);
 	cc->nr_freepages++;
+	cc->nr_migratepages++;
 }
 
 /* possible outcome of isolate_migratepages */
···
 	unsigned long last_migrated_pfn;
 	const bool sync = cc->mode != MIGRATE_ASYNC;
 	bool update_cached;
-	unsigned int nr_succeeded = 0;
+	unsigned int nr_succeeded = 0, nr_migratepages;
 
 	/*
 	 * These counters track activities during zone compaction.  Initialize
···
 				pageblock_start_pfn(cc->migrate_pfn - 1));
 		}
 
+		/*
+		 * Record the number of pages to migrate since the
+		 * compaction_alloc/free() will update cc->nr_migratepages
+		 * properly.
+		 */
+		nr_migratepages = cc->nr_migratepages;
 		err = migrate_pages(&cc->migratepages, compaction_alloc,
 				compaction_free, (unsigned long)cc, cc->mode,
 				MR_COMPACTION, &nr_succeeded);
 
-		trace_mm_compaction_migratepages(cc, nr_succeeded);
+		trace_mm_compaction_migratepages(nr_migratepages, nr_succeeded);
 
 		/* All pages were either migrated or will be released */
 		cc->nr_migratepages = 0;