[PATCH 27/27] mm: vmstat: Account per-zone stalls and pages skipped during reclaim

Mel Gorman
2016-04-15T09:18:30Z

The vmstat allocstall was fairly useful in the general sense but
node-based LRUs change that. It's important to know if a stall was for an
address-limited allocation request as this will require skipping pages from
other zones. This patch adds pgstall_* counters to replace allocstall. The
sum of the counters will equal the old allocstall so it can be trivially
recalculated. A high number of address-limited allocation requests may
result in a lot of useless LRU scanning for suitable pages.

As address-limited allocations require pages to be skipped, it's important
to know how much useless LRU scanning took place so this patch adds
pgskip* counters. This yields the following model

1. The number of address-space limited stalls can be accounted for (pgstall)
2. The amount of useless work required to reclaim the data is accounted (pgskip)
3. The total number of scans is available from pgscan_kswapd and pgscan_direct
so from that the ratio of useful to useless scans can be calculated.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
include/linux/vm_event_item.h | 4 +++-
mm/vmscan.c | 15 +++++++++++++--
mm/vmstat.c | 3 ++-
3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 8dcb5a813163..0a0503da8c3b 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -23,6 +23,8 @@

enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
FOR_ALL_ZONES(PGALLOC),
+ FOR_ALL_ZONES(PGSTALL),
+ FOR_ALL_ZONES(PGSCAN_SKIP),
PGFREE, PGACTIVATE, PGDEACTIVATE,
PGFAULT, PGMAJFAULT,
PGLAZYFREED,
@@ -37,7 +39,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
#endif
PGINODESTEAL, SLABS_SCANNED, KSWAPD_INODESTEAL,
KSWAPD_LOW_WMARK_HIT_QUICKLY, KSWAPD_HIGH_WMARK_HIT_QUICKLY,
- PAGEOUTRUN, ALLOCSTALL, PGROTATED,
+ PAGEOUTRUN, PGROTATED,
DROP_PAGECACHE, DROP_SLAB,
#ifdef CONFIG_NUMA_BALANCING
NUMA_PTE_UPDATES,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index e5aa605da6c4..752990878108 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1372,6 +1372,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
struct list_head *src = &lruvec->lists[lru];
unsigned long nr_taken = 0;
unsigned long scan;
+ unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
LIST_HEAD(pages_skipped);

for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
@@ -1386,6 +1387,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,

if (page_zonenum(page) > sc->reclaim_idx) {
list_move(&page->lru, &pages_skipped);
+ nr_skipped[page_zonenum(page)]++;
continue;
}

@@ -1414,8 +1416,17 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
* scanning would soon rescan the same pages to skip and put the
* system at risk of premature OOM.
*/
- if (!list_empty(&pages_skipped))
+ if (!list_empty(&pages_skipped)) {
+ int zid;
+
list_splice(&pages_skipped, src);
+ for (zid = 0; zid < MAX_NR_ZONES; zid++) {
+ if (!nr_skipped[zid])
+ continue;
+
+ __count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
+ }
+ }
*nr_scanned = scan;
trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,
nr_taken, mode, is_file_lru(lru));
@@ -2684,7 +2695,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
delayacct_freepages_start();

if (global_reclaim(sc))
- count_vm_event(ALLOCSTALL);
+ __count_zid_vm_events(PGSTALL, classzone_idx, 1);

do {
vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 7ae67218d675..db129e751eb9 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -969,6 +969,8 @@ const char * const vmstat_text[] = {
"pswpout",

TEXTS_FOR_ZONES("pgalloc")
+ TEXTS_FOR_ZONES("pgstall")
+ TEXTS_FOR_ZONES("pgskip")

"pgfree",
"pgactivate",
@@ -994,7 +996,6 @@ const char * const vmstat_text[] = {
"kswapd_low_wmark_hit_quickly",
"kswapd_high_wmark_hit_quickly",
"pageoutrun",
- "allocstall",

"pgrotated",

--
2.6.4