From: Hugh Dickins <hughd@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Roman Gushchin <guro@fb.com>, Hugh Dickins <hughd@google.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Vlastimil Babka <vbabka@suse.cz>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats
Date: Mon, 1 Mar 2021 22:03:26 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.2.11.2103012158540.7549@eggly.anvils> (raw)
In-Reply-To: <YD2IGxfOE5AgYuzc@carbon.dhcp.thefacebook.com>
vmstat_refresh() can occasionally catch nr_zone_write_pending and
nr_writeback when they are transiently negative. The reason is partly
that the interrupt which decrements them in test_clear_page_writeback()
can come in before __test_set_page_writeback() got to increment them;
but transient negatives are still seen even when that is prevented, and
I am not yet certain why (but see Roman's note below). Those stats are
not buggy, they have never been seen to drift away from 0 permanently:
so just avoid the annoyance of showing a warning on them.
Similarly avoid showing a warning on nr_free_cma: CMA users have seen
that one reported negative from /proc/sys/vm/stat_refresh too, but it
does drift away permanently: I believe that's because its incrementation
and decrementation are decided by page migratetype, but the migratetype
of a pageblock is not guaranteed to be constant.
Roman Gushchin points out:
For performance reasons, vmstat counters are incremented and decremented
using per-cpu batches. vmstat_refresh() flushes the per-cpu batches on
all CPUs, to get values as accurate as possible; but this method is not
atomic, so the resulting value is not always precise. As a consequence,
for those counters whose actual value is close to 0, a small negative
value may occasionally be reported. If the value is small and the state
is transient, it is not an indication of an error.
Link: https://lore.kernel.org/linux-mm/20200714173747.3315771-1-guro@fb.com/
Reported-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/vmstat.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- vmstat2/mm/vmstat.c 2021-02-25 11:56:18.000000000 -0800
+++ vmstat3/mm/vmstat.c 2021-02-25 12:42:15.000000000 -0800
@@ -1840,6 +1840,14 @@ int vmstat_refresh(struct ctl_table *tab
if (err)
return err;
for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
+ /*
+ * Skip checking stats known to go negative occasionally.
+ */
+ switch (i) {
+ case NR_ZONE_WRITE_PENDING:
+ case NR_FREE_CMA_PAGES:
+ continue;
+ }
val = atomic_long_read(&vm_zone_stat[i]);
if (val < 0) {
pr_warn("%s: %s %ld\n",
@@ -1856,6 +1864,13 @@ int vmstat_refresh(struct ctl_table *tab
}
#endif
for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
+ /*
+ * Skip checking stats known to go negative occasionally.
+ */
+ switch (i) {
+ case NR_WRITEBACK:
+ continue;
+ }
val = atomic_long_read(&vm_node_stat[i]);
if (val < 0) {
pr_warn("%s: %s %ld\n",
next prev parent reply other threads:[~2021-03-02 8:24 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-25 23:10 [PATCH 1/4] mm: restore node stat checking in /proc/sys/vm/stat_refresh Hugh Dickins
2021-02-25 23:12 ` [PATCH 2/4] mm: no more EINVAL from /proc/sys/vm/stat_refresh Hugh Dickins
2021-03-01 0:38 ` Roman Gushchin
2021-02-25 23:14 ` [PATCH 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats Hugh Dickins
2021-03-01 0:53 ` Roman Gushchin
2021-03-01 22:08 ` Hugh Dickins
2021-03-02 0:34 ` Roman Gushchin
2021-03-02 6:03 ` Hugh Dickins [this message]
2021-03-04 2:25 ` [PATCH v2 " Roman Gushchin
2021-02-25 23:15 ` [PATCH 4/4] mm: /proc//sys/vm/stat_refresh stop checking monotonic numa stats Hugh Dickins
2021-03-01 0:53 ` Roman Gushchin
2021-03-01 0:37 ` [PATCH 1/4] mm: restore node stat checking in /proc/sys/vm/stat_refresh Roman Gushchin
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