mm: /proc/sys/vm/stat_refresh to force vmstat update
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>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Yang Shi <yang.shi@linaro.org>
Cc: Ning Qu <quning@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a7de9ad..c831be3 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1379,6 +1379,66 @@
int sysctl_stat_interval __read_mostly = HZ;
static cpumask_var_t cpu_stat_off;
+#ifdef CONFIG_PROC_FS
+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;
+}
+#endif /* CONFIG_PROC_FS */
+
static void vmstat_update(struct work_struct *w)
{
if (refresh_cpu_vm_stats(true)) {