[patch 2/3] MM: allow per-cpu vmstat_threshold configuration

From: Marcelo Tosatti
Date: Wed May 03 2017 - 14:45:12 EST

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The per-CPU vmstat worker is a problem on -RT workloads (because
ideally the CPU is entirely reserved for the -RT app, without
interference). The worker transfers accumulated per-CPU
vmstat counters to global counters.

To resolve the problem, create a userspace configurable per-CPU
vmstat threshold: by default the VM code calculates the size of
the per-CPU vmstat arrays. This tunable allows userspace to
configure the vmstat threshold values.

The patch below contains documentation which describes the tunables
in more detail.

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

---
Documentation/vm/vmstat_thresholds.txt | 78 +++++++++++++
mm/vmstat.c | 188 +++++++++++++++++++++++++++++----
2 files changed, 247 insertions(+), 19 deletions(-)

Index: linux-2.6-git-disable-vmstat-worker/mm/vmstat.c
===================================================================
--- linux-2.6-git-disable-vmstat-worker.orig/mm/vmstat.c 2017-04-25 07:39:13.941019853 -0300
+++ linux-2.6-git-disable-vmstat-worker/mm/vmstat.c 2017-05-03 10:59:43.495714336 -0300
@@ -91,8 +91,16 @@
EXPORT_SYMBOL(vm_zone_stat);
EXPORT_SYMBOL(vm_node_stat);

+struct vmstat_uparam {
+ atomic_t user_stat_thresh;
+};
+
+static DEFINE_PER_CPU(struct vmstat_uparam, vmstat_uparam);
+
#ifdef CONFIG_SMP

+#define MAX_THRESHOLD 125
+
int calculate_pressure_threshold(struct zone *zone)
{
int threshold;
@@ -110,9 +118,9 @@
threshold = max(1, (int)(watermark_distance / num_online_cpus()));

/*
- * Maximum threshold is 125
+ * Maximum threshold is MAX_THRESHOLD == 125
*/
- threshold = min(125, threshold);
+ threshold = min(MAX_THRESHOLD, threshold);

return threshold;
}
@@ -188,15 +196,31 @@
threshold = calculate_normal_threshold(zone);

for_each_online_cpu(cpu) {
- int pgdat_threshold;
+ int pgdat_threshold, ustat_thresh;
+ struct vmstat_uparam *vup;

- per_cpu_ptr(zone->pageset, cpu)->stat_threshold
- = threshold;
+ struct per_cpu_nodestat __percpu *pcp;
+ struct per_cpu_pageset *p;
+
+ p = per_cpu_ptr(zone->pageset, cpu);
+
+ vup = &per_cpu(vmstat_uparam, cpu);
+ ustat_thresh = atomic_read(&vup->user_stat_thresh);
+
+ if (ustat_thresh)
+ p->stat_threshold = ustat_thresh;
+ else
+ p->stat_threshold = threshold;
+
+ pcp = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);

/* Base nodestat threshold on the largest populated zone. */
- pgdat_threshold = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu)->stat_threshold;
- per_cpu_ptr(pgdat->per_cpu_nodestats, cpu)->stat_threshold
- = max(threshold, pgdat_threshold);
+ pgdat_threshold = pcp->stat_threshold;
+ if (ustat_thresh)
+ pcp->stat_threshold = ustat_thresh;
+ else
+ pcp->stat_threshold = max(threshold,
+ pgdat_threshold);
}

/*
@@ -226,9 +250,24 @@
continue;

threshold = (*calculate_pressure)(zone);
- for_each_online_cpu(cpu)
+ for_each_online_cpu(cpu) {
+ int t, ustat_thresh;
+ struct vmstat_uparam *vup;
+
+ vup = &per_cpu(vmstat_uparam, cpu);
+ ustat_thresh = atomic_read(&vup->user_stat_thresh);
+ t = threshold;
+
+ /*
+ * min because pressure could cause
+ * calculate_pressure'ed value to be smaller.
+ */
+ if (ustat_thresh)
+ t = min(threshold, ustat_thresh);
+
per_cpu_ptr(zone->pageset, cpu)->stat_threshold
- = threshold;
+ = t;
+ }
}
}

@@ -249,7 +288,7 @@

t = __this_cpu_read(pcp->stat_threshold);

- if (unlikely(x > t || x < -t)) {
+ if (unlikely(x >= t || x <= -t)) {
zone_page_state_add(x, zone, item);
x = 0;
}
@@ -269,7 +308,7 @@

t = __this_cpu_read(pcp->stat_threshold);

- if (unlikely(x > t || x < -t)) {
+ if (unlikely(x >= t || x <= -t)) {
node_page_state_add(x, pgdat, item);
x = 0;
}
@@ -308,7 +347,7 @@

v = __this_cpu_inc_return(*p);
t = __this_cpu_read(pcp->stat_threshold);
- if (unlikely(v > t)) {
+ if (unlikely(v >= t)) {
s8 overstep = t >> 1;

zone_page_state_add(v + overstep, zone, item);
@@ -324,7 +363,7 @@

v = __this_cpu_inc_return(*p);
t = __this_cpu_read(pcp->stat_threshold);
- if (unlikely(v > t)) {
+ if (unlikely(v >= t)) {
s8 overstep = t >> 1;

node_page_state_add(v + overstep, pgdat, item);
@@ -352,7 +391,7 @@

v = __this_cpu_dec_return(*p);
t = __this_cpu_read(pcp->stat_threshold);
- if (unlikely(v < - t)) {
+ if (unlikely(v <= - t)) {
s8 overstep = t >> 1;

zone_page_state_add(v - overstep, zone, item);
@@ -368,7 +407,7 @@

v = __this_cpu_dec_return(*p);
t = __this_cpu_read(pcp->stat_threshold);
- if (unlikely(v < - t)) {
+ if (unlikely(v <= - t)) {
s8 overstep = t >> 1;

node_page_state_add(v - overstep, pgdat, item);
@@ -426,7 +465,7 @@
o = this_cpu_read(*p);
n = delta + o;

- if (n > t || n < -t) {
+ if (n >= t || n <= -t) {
int os = overstep_mode * (t >> 1) ;

/* Overflow must be added to zone counters */
@@ -483,7 +522,7 @@
o = this_cpu_read(*p);
n = delta + o;

- if (n > t || n < -t) {
+ if (n >= t || n <= -t) {
int os = overstep_mode * (t >> 1) ;

/* Overflow must be added to node counters */
@@ -1696,6 +1735,96 @@
round_jiffies_relative(sysctl_stat_interval));
}

+#ifdef CONFIG_SYSFS
+
+static ssize_t vmstat_thresh_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ int ret;
+ struct vmstat_uparam *vup;
+ unsigned int cpu = dev->id;
+
+ preempt_disable();
+
+ vup = &per_cpu(vmstat_uparam, cpu);
+ ret = sprintf(buf, "%d\n", atomic_read(&vup->user_stat_thresh));
+
+ preempt_enable();
+
+ return ret;
+}
+
+static ssize_t vmstat_thresh_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ int ret, val;
+ unsigned int cpu = dev->id;
+ struct vmstat_uparam *vup;
+
+ ret = sscanf(buf, "%d", &val);
+ if (ret != 1 || val < 1 || val > MAX_THRESHOLD)
+ return -EINVAL;
+
+ preempt_disable();
+
+ if (cpu_online(cpu)) {
+ vup = &per_cpu(vmstat_uparam, cpu);
+ atomic_set(&vup->user_stat_thresh, val);
+ } else
+ count = -EINVAL;
+
+ preempt_enable();
+
+ return count;
+}
+
+struct device_attribute vmstat_threshold_attr =
+ __ATTR(vmstat_threshold, 0644, vmstat_thresh_show, vmstat_thresh_store);
+
+static struct attribute *vmstat_attrs[] = {
+ &vmstat_threshold_attr.attr,
+ NULL
+};
+
+static struct attribute_group vmstat_attr_group = {
+ .attrs = vmstat_attrs,
+ .name = "vmstat"
+};
+
+static int vmstat_thresh_cpu_online(unsigned int cpu)
+{
+ struct device *dev = get_cpu_device(cpu);
+ int ret;
+
+ ret = sysfs_create_group(&dev->kobj, &vmstat_attr_group);
+ if (ret)
+ return ret;
+
+ return 0;
+}
+
+static int vmstat_thresh_cpu_down_prep(unsigned int cpu)
+{
+ struct device *dev = get_cpu_device(cpu);
+
+ sysfs_remove_group(&dev->kobj, &vmstat_attr_group);
+ return 0;
+}
+
+static void init_vmstat_sysfs(void)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ struct vmstat_uparam *vup = &per_cpu(vmstat_uparam, cpu);
+
+ atomic_set(&vup->user_stat_thresh, 0);
+ }
+}
+
+#endif /* CONFIG_SYSFS */
+
static void __init init_cpu_node_state(void)
{
int node;
@@ -1723,9 +1852,12 @@
{
const struct cpumask *node_cpus;
int node;
+ struct vmstat_uparam *vup = &per_cpu(vmstat_uparam, cpu);

node = cpu_to_node(cpu);

+ atomic_set(&vup->user_stat_thresh, 0);
+
refresh_zone_stat_thresholds();
node_cpus = cpumask_of_node(node);
if (cpumask_weight(node_cpus) > 0)
@@ -1735,7 +1867,7 @@
return 0;
}

-#endif
+#endif /* CONFIG_SMP */

struct workqueue_struct *mm_percpu_wq;

@@ -1772,6 +1904,24 @@
#endif
}

+static int __init init_mm_internals_late(void)
+{
+#ifdef CONFIG_SYSFS
+ int ret;
+
+ init_vmstat_sysfs();
+
+ ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mm/vmstat_thresh:online",
+ vmstat_thresh_cpu_online,
+ vmstat_thresh_cpu_down_prep);
+ if (ret < 0)
+ pr_err("vmstat_thresh: failed to register 'online' hotplug state\n");
+#endif
+ return 0;
+}
+
+late_initcall(init_mm_internals_late);
+
#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_COMPACTION)

/*
Index: linux-2.6-git-disable-vmstat-worker/Documentation/vm/vmstat_thresholds.txt
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-git-disable-vmstat-worker/Documentation/vm/vmstat_thresholds.txt 2017-05-02 13:48:45.946840708 -0300
@@ -0,0 +1,78 @@
+Userspace configurable vmstat thresholds
+========================================
+
+This document describes the tunables to control
+per-CPU vmstat threshold and per-CPU vmstat worker
+thread.
+
+/sys/devices/system/cpu/cpuN/vmstat/vmstat_threshold:
+
+This file contains the per-CPU vmstat threshold.
+This value is the maximum that a single per-CPU vmstat statistic
+can accumulate before transferring to the global counters.
+
+A value of 0 indicates that the value is set
+by the in kernel algorithm.
+
+A value different than 0 indicates that particular
+value is used for vmstat_threshold.
+
+/sys/devices/system/cpu/cpuN/vmstat/vmstat_worker:
+
+Enable/disable the per-CPU vmstat worker.
+
+What does the vmstat_threshold value mean? What are the implications
+of changing this value? What's the difference in choosing 1, 2, 3
+or 500?
+====================================================================
+
+Its the maximum value for a vmstat statistics counter to hold. After
+that value, the statistics are transferred to the global counter:
+
+void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
+ long delta)
+{
+ struct per_cpu_nodestat __percpu *pcp = pgdat->per_cpu_nodestats;
+ s8 __percpu *p = pcp->vm_node_stat_diff + item;
+ long x;
+ long t;
+
+ x = delta + __this_cpu_read(*p);
+
+ t = __this_cpu_read(pcp->stat_threshold);
+
+ if (unlikely(x > t || x < -t)) {
+ node_page_state_add(x, pgdat, item);
+ x = 0;
+ }
+ __this_cpu_write(*p, x);
+}
+
+Increasing the threshold value does two things:
+ 1) It decreases the number of inter-processor accesses.
+ 2) It increases how much the global counters stay out of
+ sync relative to actual current values.
+
+
+Usage example:
+=============
+
+In a realtime system, the worker thread waking up and executing
+vmstat_update can be an undesired source of latencies.
+
+To avoid the worker thread from waking up, executing vmstat_update
+on cpu 1, for example, perform the following steps:
+
+
+cd /sys/devices/system/cpu/cpu0/vmstat/
+
+# Set vmstat threshold to 1 for cpu1, so that no
+# vmstat statistics are collected in cpu1's per-cpu
+# stats, instead they are immediately transferred
+# to the global counter.
+
+$ echo 1 > vmstat_threshold
+
+# Disable vmstat_update worker for cpu1:
+$ echo 0 > vmstat_worker
+

Next message: Marcelo Tosatti: "[patch 1/3] MM: remove unused quiet_vmstat function"
Previous message: Dan Williams: "Re: [PATCH] brd: fix uninitialized use of brd->dax_dev"
In reply to: Marcelo Tosatti: "[patch 0/3] per-CPU vmstat thresholds and vmstat worker disablement (v2)"
Next in thread: Marcelo Tosatti: "[patch 1/3] MM: remove unused quiet_vmstat function"
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