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>
Change-Id: Ic5d6e84bc093b2483e13a755710a25fa0f8a0cf1
Git-commit: c73322d098e4b6f5f0f0fa1330bf57e218775539
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7b5848cf..d0d1763 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2704,6 +2704,15 @@
} 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;
}
@@ -2778,10 +2787,6 @@
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.
@@ -2918,7 +2923,7 @@
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;
@@ -2926,6 +2931,9 @@
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) ||
@@ -3006,7 +3014,7 @@
/* Throttle based on the first usable node */
pgdat = zone->zone_pgdat;
- if (pfmemalloc_watermark_ok(pgdat))
+ if (allow_direct_reclaim(pgdat))
goto out;
break;
}
@@ -3028,14 +3036,14 @@
*/
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))
@@ -3214,7 +3222,7 @@
/*
* 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
@@ -3228,6 +3236,10 @@
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;
@@ -3314,9 +3326,9 @@
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;
/*
@@ -3395,7 +3407,7 @@
* 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 */
@@ -3406,10 +3418,14 @@
* 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
@@ -3609,6 +3625,10 @@
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;
@@ -3879,9 +3899,6 @@
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.
*/