mm: vmstat: fix /proc/sys/vm/stat_refresh generating false warnings

I've noticed a number of warnings like "vmstat_refresh: nr_free_cma -5" or
"vmstat_refresh: nr_zone_write_pending -11" on our production hosts.  The
numbers of these warnings were relatively low and stable, so it didn't
look like we are systematically leaking the counters.  The corresponding
vmstat counters also looked sane.

These warnings are generated by the vmstat_refresh() function, which
assumes that atomic zone and numa counters can't go below zero.  However,
on a SMP machine it's not quite right: due to per-cpu caching it can in
theory be as low as -(zone threshold) * NR_CPUs.

For instance, let's say all cma pages are in use and NR_FREE_CMA_PAGES
reached 0.  Then we've reclaimed a small number of cma pages on each CPU
except CPU0, so that most percpu NR_FREE_CMA_PAGES counters are slightly
positive (the atomic counter is still 0).  Then somebody on CPU0 consumes
all these pages.  The number of pages can easily exceed the threshold and
a negative value will be committed to the atomic counter.

To fix the problem and avoid generating false warnings, let's just relax
the condition and warn only if the value is less than minus the maximum
theoretically possible drift value, which is 125 * number of online CPUs.
It will still allow to catch systematic leaks, but will not generate bogus
warnings.

Link: http://lkml.kernel.org/r/20200714173920.3319063-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index e35a3f2fb006adb1fd9c7e70a4005da4f834d98b..a604bf5825b8e1d6501b6a66bbcd15fe8249f27d 100644 (file)
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -824,8 +824,8 @@ e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
 
 As a side-effect, it also checks for negative totals (elsewhere reported
 as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
-(At time of writing, a few stats are known sometimes to be found negative,
-with no ill effects: errors and warnings on these stats are suppressed.)
+(On a SMP machine some stats can temporarily become negative, with no ill
+effects: errors and warnings on these stats are suppressed.)
 
 
 numa_stat

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 07dc0af50cf03f237afc2a3bfee8d774f9e1f67e..69c3ae37c97d6c2e4808b0844d534bff0f7c888d 100644 (file)
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -169,6 +169,8 @@ EXPORT_SYMBOL(vm_node_stat);
 
 #ifdef CONFIG_SMP
 
+#define MAX_THRESHOLD 125
+
 int calculate_pressure_threshold(struct zone *zone)
 {
        int threshold;
@@ -186,11 +188,9 @@ int calculate_pressure_threshold(struct zone *zone)
        threshold = max(1, (int)(watermark_distance / num_online_cpus()));
 
        /*
-        * Maximum threshold is 125
+        * Threshold is capped by MAX_THRESHOLD
         */
-       threshold = min(125, threshold);
-
-       return threshold;
+       return min(MAX_THRESHOLD, threshold);
 }
 
 int calculate_normal_threshold(struct zone *zone)
@@ -610,6 +610,9 @@ void dec_node_page_state(struct page *page, enum node_stat_item item)
 }
 EXPORT_SYMBOL(dec_node_page_state);
 #else
+
+#define MAX_THRESHOLD 0
+
 /*
  * Use interrupt disable to serialize counter updates
  */
@@ -1817,7 +1820,7 @@ static void refresh_vm_stats(struct work_struct *work)
 int vmstat_refresh(struct ctl_table *table, int write,
                   void *buffer, size_t *lenp, loff_t *ppos)
 {
-       long val;
+       long val, max_drift;
        int err;
        int i;
 
@@ -1828,17 +1831,22 @@ int vmstat_refresh(struct ctl_table *table, int write,
         * pages, immediately after running a test.  /proc/sys/vm/stat_refresh,
         * which can equally be echo'ed to or cat'ted from (by root),
         * can be used to update the stats just before reading them.
-        *
-        * Oh, and since global_zone_page_state() etc. are so careful to hide
-        * transiently negative values, report an error here if any of
-        * the stats is negative, so we know to go looking for imbalance.
         */
        err = schedule_on_each_cpu(refresh_vm_stats);
        if (err)
                return err;
+
+       /*
+        * Since global_zone_page_state() etc. are so careful to hide
+        * transiently negative values, report an error here if any of
+        * the stats is negative and are less than the maximum drift value,
+        * so we know to go looking for imbalance.
+        */
+       max_drift = num_online_cpus() * MAX_THRESHOLD;
+
        for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
                val = atomic_long_read(&vm_zone_stat[i]);
-               if (val < 0) {
+               if (val < -max_drift) {
                        pr_warn("%s: %s %ld\n",
                                __func__, zone_stat_name(i), val);
                        err = -EINVAL;
@@ -1847,7 +1855,7 @@ int vmstat_refresh(struct ctl_table *table, int write,
 #ifdef CONFIG_NUMA
        for (i = 0; i < NR_VM_NUMA_STAT_ITEMS; i++) {
                val = atomic_long_read(&vm_numa_stat[i]);
-               if (val < 0) {
+               if (val < -max_drift) {
                        pr_warn("%s: %s %ld\n",
                                __func__, numa_stat_name(i), val);
                        err = -EINVAL;