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/6440493f18da82298152b6305d6b41c2962a3ce6.1708409245.git.baolin.wang@linux.alibaba.comSigned-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/trace/events/compaction.h

@@ -78,10 +78,10 @@ DEFINE_EVENT(mm_compaction_isolate_template, mm_compaction_fast_isolate_freepage
 #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)
@@ -90,7 +90,7 @@ TRACE_EVENT(mm_compaction_migratepages,
 
 	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

@@ -1796,6 +1796,7 @@ 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--;
 
 	return dst;
 }
@@ -1811,6 +1812,7 @@ 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 */
@@ -2433,7 +2435,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 	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
@@ -2551,11 +2553,17 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 				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;