[PATCH 06/10] mm: /proc/sys/vm/stat_refresh to force vmstat update

Hugh Dickins
2016-04-05T20:49:40Z

Provide /proc/sys/vm/stat_refresh to force an immediate update of
per-cpu into global vmstats: useful to avoid a sleep(2) or whatever
before checking counts when testing. Originally added to work around
a bug which left counts stranded indefinitely on a cpu going idle
(an inaccuracy magnified when small below-batch numbers represent
"huge" amounts of memory), but I believe that bug is now fixed:
nonetheless, this is still a useful knob.

Its schedule_on_each_cpu() is probably too expensive just to fold
into reading /proc/meminfo itself: give this mode 0600 to prevent
abuse. Allow a write or a read to do the same: nothing to read,
but "grep -h Shmem /proc/sys/vm/stat_refresh /proc/meminfo" is
convenient. Oh, and since global_page_state() itself is careful
to disguise any underflow as 0, hack in an "Invalid argument" and
pr_warn() if a counter is negative after the refresh - this helped
to fix a misaccounting of NR_ISOLATED_FILE in my migration code.

But on recent kernels, I find that NR_ALLOC_BATCH and NR_PAGES_SCANNED
often go negative some of the time. I have not yet worked out why, but
have no evidence that it's actually harmful. Punt for the moment by
just ignoring the anomaly on those.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
Documentation/sysctl/vm.txt | 14 ++++++++
include/linux/vmstat.h | 4 ++
kernel/sysctl.c | 7 ++++
mm/vmstat.c | 58 ++++++++++++++++++++++++++++++++++
4 files changed, 83 insertions(+)

--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -57,6 +57,7 @@ Currently, these files are in /proc/sys/
- panic_on_oom
- percpu_pagelist_fraction
- stat_interval
+- stat_refresh
- swappiness
- user_reserve_kbytes
- vfs_cache_pressure
@@ -754,6 +755,19 @@ is 1 second.

==============================================================

+stat_refresh
+
+Any read or write (by root only) flushes all the per-cpu vm statistics
+into their global totals, for more accurate reports when testing
+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.)
+
+==============================================================
+
swappiness

This control is used to define how aggressive the kernel will swap
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -193,6 +193,10 @@ void quiet_vmstat(void);
void cpu_vm_stats_fold(int cpu);
void refresh_zone_stat_thresholds(void);

+struct ctl_table;
+int vmstat_refresh(struct ctl_table *, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos);
+
void drain_zonestat(struct zone *zone, struct per_cpu_pageset *);

int calculate_pressure_threshold(struct zone *zone);
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1509,6 +1509,13 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec_jiffies,
},
+ {
+ .procname = "stat_refresh",
+ .data = NULL,
+ .maxlen = 0,
+ .mode = 0600,
+ .proc_handler = vmstat_refresh,
+ },
#endif
#ifdef CONFIG_MMU
{
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1378,6 +1378,64 @@ static DEFINE_PER_CPU(struct delayed_wor
int sysctl_stat_interval __read_mostly = HZ;
static cpumask_var_t cpu_stat_off;

+static void refresh_vm_stats(struct work_struct *work)
+{
+ refresh_cpu_vm_stats(true);
+}
+
+int vmstat_refresh(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ long val;
+ int err;
+ int i;
+
+ /*
+ * The regular update, every sysctl_stat_interval, may come later
+ * than expected: leaving a significant amount in per_cpu buckets.
+ * This is particularly misleading when checking a quantity of HUGE
+ * 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_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;
+ for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
+ val = atomic_long_read(&vm_stat[i]);
+ if (val < 0) {
+ switch (i) {
+ case NR_ALLOC_BATCH:
+ case NR_PAGES_SCANNED:
+ /*
+ * These are often seen to go negative in
+ * recent kernels, but not to go permanently
+ * negative. Whilst it would be nicer not to
+ * have exceptions, rooting them out would be
+ * another task, of rather low priority.
+ */
+ break;
+ default:
+ pr_warn("%s: %s %ld\n",
+ __func__, vmstat_text[i], val);
+ err = -EINVAL;
+ break;
+ }
+ }
+ }
+ if (err)
+ return err;
+ if (write)
+ *ppos += *lenp;
+ else
+ *lenp = 0;
+ return 0;
+}
+
static void vmstat_update(struct work_struct *w)
{
if (refresh_cpu_vm_stats(true)) {