mm: vmscan: split khugepaged stats from direct reclaim stats

Direct reclaim stats are useful for identifying a potential source for
application latency, as well as spotting issues with kswapd.  However,
khugepaged currently distorts the picture: as a kernel thread it doesn't
impose allocation latencies on userspace, and it explicitly opts out of
kswapd reclaim.  Its activity showing up in the direct reclaim stats is
misleading.  Counting it as kswapd reclaim could also cause confusion when
trying to understand actual kswapd behavior.

Break out khugepaged from the direct reclaim counters into new
pgsteal_khugepaged, pgdemote_khugepaged, pgscan_khugepaged counters.

Test with a huge executable (CONFIG_READ_ONLY_THP_FOR_FS):

pgsteal_kswapd 1342185
pgsteal_direct 0
pgsteal_khugepaged 3623
pgscan_kswapd 1345025
pgscan_direct 0
pgscan_khugepaged 3623

Link: https://lkml.kernel.org/r/20221026180133.377671-1-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Eric Bergen <ebergen@meta.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/cgroup-v2.rst

@@ -1488,12 +1488,18 @@ PAGE_SIZE multiple when read back.
 	  pgscan_direct (npn)
 		Amount of scanned pages directly  (in an inactive LRU list)
 
+	  pgscan_khugepaged (npn)
+		Amount of scanned pages by khugepaged  (in an inactive LRU list)
+
 	  pgsteal_kswapd (npn)
 		Amount of reclaimed pages by kswapd
 
 	  pgsteal_direct (npn)
 		Amount of reclaimed pages directly
 
+	  pgsteal_khugepaged (npn)
+		Amount of reclaimed pages by khugepaged
+
 	  pgfault (npn)
 		Total number of page faults incurred
 

include/linux/khugepaged.h

@@ -15,6 +15,7 @@ extern void __khugepaged_exit(struct mm_struct *mm);
 extern void khugepaged_enter_vma(struct vm_area_struct *vma,
 				 unsigned long vm_flags);
 extern void khugepaged_min_free_kbytes_update(void);
+extern bool current_is_khugepaged(void);
 #ifdef CONFIG_SHMEM
 extern int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 				   bool install_pmd);
@@ -57,6 +58,11 @@ static inline int collapse_pte_mapped_thp(struct mm_struct *mm,
 static inline void khugepaged_min_free_kbytes_update(void)
 {
 }
+
+static inline bool current_is_khugepaged(void)
+{
+	return false;
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_KHUGEPAGED_H */

include/linux/vm_event_item.h

@@ -40,10 +40,13 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		PGREUSE,
 		PGSTEAL_KSWAPD,
 		PGSTEAL_DIRECT,
+		PGSTEAL_KHUGEPAGED,
 		PGDEMOTE_KSWAPD,
 		PGDEMOTE_DIRECT,
+		PGDEMOTE_KHUGEPAGED,
 		PGSCAN_KSWAPD,
 		PGSCAN_DIRECT,
+		PGSCAN_KHUGEPAGED,
 		PGSCAN_DIRECT_THROTTLE,
 		PGSCAN_ANON,
 		PGSCAN_FILE,

mm/khugepaged.c

@@ -2577,6 +2577,11 @@ void khugepaged_min_free_kbytes_update(void)
 	mutex_unlock(&khugepaged_mutex);
 }
 
+bool current_is_khugepaged(void)
+{
+	return kthread_func(current) == khugepaged;
+}
+
 static int madvise_collapse_errno(enum scan_result r)
 {
 	/*

mm/memcontrol.c

@@ -661,8 +661,10 @@ static const unsigned int memcg_vm_event_stat[] = {
 	PGPGOUT,
 	PGSCAN_KSWAPD,
 	PGSCAN_DIRECT,
+	PGSCAN_KHUGEPAGED,
 	PGSTEAL_KSWAPD,
 	PGSTEAL_DIRECT,
+	PGSTEAL_KHUGEPAGED,
 	PGFAULT,
 	PGMAJFAULT,
 	PGREFILL,
@@ -1574,10 +1576,12 @@ static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
 	/* Accumulated memory events */
 	seq_buf_printf(&s, "pgscan %lu\n",
 		       memcg_events(memcg, PGSCAN_KSWAPD) +
-		       memcg_events(memcg, PGSCAN_DIRECT));
+		       memcg_events(memcg, PGSCAN_DIRECT) +
+		       memcg_events(memcg, PGSCAN_KHUGEPAGED));
 	seq_buf_printf(&s, "pgsteal %lu\n",
 		       memcg_events(memcg, PGSTEAL_KSWAPD) +
-		       memcg_events(memcg, PGSTEAL_DIRECT));
+		       memcg_events(memcg, PGSTEAL_DIRECT) +
+		       memcg_events(memcg, PGSTEAL_KHUGEPAGED));
 
 	for (i = 0; i < ARRAY_SIZE(memcg_vm_event_stat); i++) {
 		if (memcg_vm_event_stat[i] == PGPGIN ||

mm/vmscan.c

@@ -54,6 +54,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/ctype.h>
 #include <linux/debugfs.h>
+#include <linux/khugepaged.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>
@@ -1047,6 +1048,24 @@ void drop_slab(void)
 		drop_slab_node(nid);
 }
 
+static int reclaimer_offset(void)
+{
+	BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD !=
+			PGDEMOTE_DIRECT - PGDEMOTE_KSWAPD);
+	BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD !=
+			PGSCAN_DIRECT - PGSCAN_KSWAPD);
+	BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD !=
+			PGDEMOTE_KHUGEPAGED - PGDEMOTE_KSWAPD);
+	BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD !=
+			PGSCAN_KHUGEPAGED - PGSCAN_KSWAPD);
+
+	if (current_is_kswapd())
+		return 0;
+	if (current_is_khugepaged())
+		return PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD;
+	return PGSTEAL_DIRECT - PGSTEAL_KSWAPD;
+}
+
 static inline int is_page_cache_freeable(struct folio *folio)
 {
 	/*
@@ -1599,10 +1618,7 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
 		      (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
 		      &nr_succeeded);
 
-	if (current_is_kswapd())
-		__count_vm_events(PGDEMOTE_KSWAPD, nr_succeeded);
-	else
-		__count_vm_events(PGDEMOTE_DIRECT, nr_succeeded);
+	__count_vm_events(PGDEMOTE_KSWAPD + reclaimer_offset(), nr_succeeded);
 
 	return nr_succeeded;
 }
@@ -2475,7 +2491,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 				     &nr_scanned, sc, lru);
 
 	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
-	item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
+	item = PGSCAN_KSWAPD + reclaimer_offset();
 	if (!cgroup_reclaim(sc))
 		__count_vm_events(item, nr_scanned);
 	__count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
@@ -2492,7 +2508,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 	move_folios_to_lru(lruvec, &folio_list);
 
 	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
-	item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
+	item = PGSTEAL_KSWAPD + reclaimer_offset();
 	if (!cgroup_reclaim(sc))
 		__count_vm_events(item, nr_reclaimed);
 	__count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
@@ -4871,7 +4887,7 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
 			break;
 	}
 
-	item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
+	item = PGSCAN_KSWAPD + reclaimer_offset();
 	if (!cgroup_reclaim(sc)) {
 		__count_vm_events(item, isolated);
 		__count_vm_events(PGREFILL, sorted);
@@ -5049,7 +5065,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
 	if (walk && walk->batched)
 		reset_batch_size(lruvec, walk);
 
-	item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
+	item = PGSTEAL_KSWAPD + reclaimer_offset();
 	if (!cgroup_reclaim(sc))
 		__count_vm_events(item, reclaimed);
 	__count_memcg_events(memcg, item, reclaimed);

mm/vmstat.c

@@ -1271,10 +1271,13 @@ const char * const vmstat_text[] = {
 	"pgreuse",
 	"pgsteal_kswapd",
 	"pgsteal_direct",
+	"pgsteal_khugepaged",
 	"pgdemote_kswapd",
 	"pgdemote_direct",
+	"pgdemote_khugepaged",
 	"pgscan_kswapd",
 	"pgscan_direct",
+	"pgscan_khugepaged",
 	"pgscan_direct_throttle",
 	"pgscan_anon",
 	"pgscan_file",