Commit 52b6f46b authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

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: default avatarHugh 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9e18eb29
......@@ -57,6 +57,7 @@ Currently, these files are in /proc/sys/vm:
- panic_on_oom
- percpu_pagelist_fraction
- stat_interval
- stat_refresh
- swappiness
- user_reserve_kbytes
- vfs_cache_pressure
......@@ -755,6 +756,19 @@ 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
......
......@@ -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);
......
......@@ -1521,6 +1521,13 @@ 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
{
......
......@@ -1379,6 +1379,66 @@ static DEFINE_PER_CPU(struct delayed_work, vmstat_work);
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)) {
......
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