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authorJohannes Weiner <hannes@cmpxchg.org>2017-05-03 14:51:51 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-05-03 15:52:07 -0700
commitc73322d098e4b6f5f0f0fa1330bf57e218775539 (patch)
tree4d03df8bef04b62be30bd02df12e118c8dfb04c3 /mm/vmscan.c
parenta87c75fbcc8dc8466be6c4b18a45bcaf315883ab (diff)
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mm: fix 100% CPU kswapd busyloop on unreclaimable nodes
Patch series "mm: kswapd spinning on unreclaimable nodes - fixes and cleanups". Jia reported a scenario in which the kswapd of a node indefinitely spins at 100% CPU usage. We have seen similar cases at Facebook. The kernel's current method of judging its ability to reclaim a node (or whether to back off and sleep) is based on the amount of scanned pages in proportion to the amount of reclaimable pages. In Jia's and our scenarios, there are no reclaimable pages in the node, however, and the condition for backing off is never met. Kswapd busyloops in an attempt to restore the watermarks while having nothing to work with. This series reworks the definition of an unreclaimable node based not on scanning but on whether kswapd is able to actually reclaim pages in MAX_RECLAIM_RETRIES (16) consecutive runs. This is the same criteria the page allocator uses for giving up on direct reclaim and invoking the OOM killer. If it cannot free any pages, kswapd will go to sleep and leave further attempts to direct reclaim invocations, which will either make progress and re-enable kswapd, or invoke the OOM killer. Patch #1 fixes the immediate problem Jia reported, the remainder are smaller fixlets, cleanups, and overall phasing out of the old method. Patch #6 is the odd one out. It's a nice cleanup to get_scan_count(), and directly related to #5, but in itself not relevant to the series. If the whole series is too ambitious for 4.11, I would consider the first three patches fixes, the rest cleanups. This patch (of 9): Jia He reports a problem with kswapd spinning at 100% CPU when requesting more hugepages than memory available in the system: $ echo 4000 >/proc/sys/vm/nr_hugepages top - 13:42:59 up 3:37, 1 user, load average: 1.09, 1.03, 1.01 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 12.5 sy, 0.0 ni, 85.5 id, 2.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem: 31371520 total, 30915136 used, 456384 free, 320 buffers KiB Swap: 6284224 total, 115712 used, 6168512 free. 48192 cached Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 76 root 20 0 0 0 0 R 100.0 0.000 217:17.29 kswapd3 At that time, there are no reclaimable pages left in the node, but as kswapd fails to restore the high watermarks it refuses to go to sleep. Kswapd needs to back away from nodes that fail to balance. Up until commit 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes") kswapd had such a mechanism. It considered zones whose theoretically reclaimable pages it had reclaimed six times over as unreclaimable and backed away from them. This guard was erroneously removed as the patch changed the definition of a balanced node. However, simply restoring this code wouldn't help in the case reported here: there *are* no reclaimable pages that could be scanned until the threshold is met. Kswapd would stay awake anyway. Introduce a new and much simpler way of backing off. If kswapd runs through MAX_RECLAIM_RETRIES (16) cycles without reclaiming a single page, make it back off from the node. This is the same number of shots direct reclaim takes before declaring OOM. Kswapd will go to sleep on that node until a direct reclaimer manages to reclaim some pages, thus proving the node reclaimable again. [hannes@cmpxchg.org: check kswapd failure against the cumulative nr_reclaimed count] Link: http://lkml.kernel.org/r/20170306162410.GB2090@cmpxchg.org [shakeelb@google.com: fix condition for throttle_direct_reclaim] Link: http://lkml.kernel.org/r/20170314183228.20152-1-shakeelb@google.com Link: http://lkml.kernel.org/r/20170228214007.5621-2-hannes@cmpxchg.org Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Shakeel Butt <shakeelb@google.com> Reported-by: Jia He <hejianet@gmail.com> Tested-by: Jia He <hejianet@gmail.com> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com> Acked-by: Minchan Kim <minchan@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/vmscan.c')
-rw-r--r--mm/vmscan.c47
1 files changed, 32 insertions, 15 deletions
diff --git a/mm/vmscan.c b/mm/vmscan.c
index bc8031ef994d..667644e53b5c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2620,6 +2620,15 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
} while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
sc->nr_scanned - nr_scanned, sc));
+ /*
+ * Kswapd gives up on balancing particular nodes after too
+ * many failures to reclaim anything from them and goes to
+ * sleep. On reclaim progress, reset the failure counter. A
+ * successful direct reclaim run will revive a dormant kswapd.
+ */
+ if (reclaimable)
+ pgdat->kswapd_failures = 0;
+
return reclaimable;
}
@@ -2694,10 +2703,6 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
GFP_KERNEL | __GFP_HARDWALL))
continue;
- if (sc->priority != DEF_PRIORITY &&
- !pgdat_reclaimable(zone->zone_pgdat))
- continue; /* Let kswapd poll it */
-
/*
* If we already have plenty of memory free for
* compaction in this zone, don't free any more.
@@ -2817,7 +2822,7 @@ retry:
return 0;
}
-static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
+static bool allow_direct_reclaim(pg_data_t *pgdat)
{
struct zone *zone;
unsigned long pfmemalloc_reserve = 0;
@@ -2825,6 +2830,9 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
int i;
bool wmark_ok;
+ if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+ return true;
+
for (i = 0; i <= ZONE_NORMAL; i++) {
zone = &pgdat->node_zones[i];
if (!managed_zone(zone) ||
@@ -2905,7 +2913,7 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
/* Throttle based on the first usable node */
pgdat = zone->zone_pgdat;
- if (pfmemalloc_watermark_ok(pgdat))
+ if (allow_direct_reclaim(pgdat))
goto out;
break;
}
@@ -2927,14 +2935,14 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
*/
if (!(gfp_mask & __GFP_FS)) {
wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
- pfmemalloc_watermark_ok(pgdat), HZ);
+ allow_direct_reclaim(pgdat), HZ);
goto check_pending;
}
/* Throttle until kswapd wakes the process */
wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
- pfmemalloc_watermark_ok(pgdat));
+ allow_direct_reclaim(pgdat));
check_pending:
if (fatal_signal_pending(current))
@@ -3114,7 +3122,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
/*
* The throttled processes are normally woken up in balance_pgdat() as
- * soon as pfmemalloc_watermark_ok() is true. But there is a potential
+ * soon as allow_direct_reclaim() is true. But there is a potential
* race between when kswapd checks the watermarks and a process gets
* throttled. There is also a potential race if processes get
* throttled, kswapd wakes, a large process exits thereby balancing the
@@ -3128,6 +3136,10 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
if (waitqueue_active(&pgdat->pfmemalloc_wait))
wake_up_all(&pgdat->pfmemalloc_wait);
+ /* Hopeless node, leave it to direct reclaim */
+ if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+ return true;
+
for (i = 0; i <= classzone_idx; i++) {
struct zone *zone = pgdat->node_zones + i;
@@ -3214,9 +3226,9 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
count_vm_event(PAGEOUTRUN);
do {
+ unsigned long nr_reclaimed = sc.nr_reclaimed;
bool raise_priority = true;
- sc.nr_reclaimed = 0;
sc.reclaim_idx = classzone_idx;
/*
@@ -3295,7 +3307,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
* able to safely make forward progress. Wake them
*/
if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
- pfmemalloc_watermark_ok(pgdat))
+ allow_direct_reclaim(pgdat))
wake_up_all(&pgdat->pfmemalloc_wait);
/* Check if kswapd should be suspending */
@@ -3306,10 +3318,14 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
* Raise priority if scanning rate is too low or there was no
* progress in reclaiming pages
*/
- if (raise_priority || !sc.nr_reclaimed)
+ nr_reclaimed = sc.nr_reclaimed - nr_reclaimed;
+ if (raise_priority || !nr_reclaimed)
sc.priority--;
} while (sc.priority >= 1);
+ if (!sc.nr_reclaimed)
+ pgdat->kswapd_failures++;
+
out:
/*
* Return the order kswapd stopped reclaiming at as
@@ -3509,6 +3525,10 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
if (!waitqueue_active(&pgdat->kswapd_wait))
return;
+ /* Hopeless node, leave it to direct reclaim */
+ if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+ return;
+
/* Only wake kswapd if all zones are unbalanced */
for (z = 0; z <= classzone_idx; z++) {
zone = pgdat->node_zones + z;
@@ -3779,9 +3799,6 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
sum_zone_node_page_state(pgdat->node_id, NR_SLAB_RECLAIMABLE) <= pgdat->min_slab_pages)
return NODE_RECLAIM_FULL;
- if (!pgdat_reclaimable(pgdat))
- return NODE_RECLAIM_FULL;
-
/*
* Do not scan if the allocation should not be delayed.
*/