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 two tunables:
* 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 values.
* Userspace configurable per-CPU vmstat worker: allow disabling
the per-CPU vmstat worker.
The patch below contains documentation which describes the tunables
in more detail.
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
---
Documentation/vm/vmstat_thresholds.txt | 38 +++++
mm/vmstat.c | 248 +++++++++++++++++++++++++++++++--
2 files changed, 272 insertions(+), 14 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 2017-04-25 10:44:51.581977296 -0300
@@ -91,8 +91,17 @@
EXPORT_SYMBOL(vm_zone_stat);
EXPORT_SYMBOL(vm_node_stat);
+struct vmstat_uparam {
+ atomic_t vmstat_work_enabled;
+ 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 +119,9 @@
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 +197,31 @@
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 +251,24 @@
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;
+ }
}
}
@@ -1567,6 +1607,9 @@
long val;
int err;
int i;
+ int cpu;
+ struct work_struct __percpu *works;
+ static struct cpumask has_work;
/*
* The regular update, every sysctl_stat_interval, may come later
@@ -1580,9 +1623,31 @@
* 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;
+
+ works = alloc_percpu(struct work_struct);
+ if (!works)
+ return -ENOMEM;
+
+ cpumask_clear(&has_work);
+ get_online_cpus();
+
+ for_each_online_cpu(cpu) {
+ struct work_struct *work = per_cpu_ptr(works, cpu);
+ struct vmstat_uparam *vup = &per_cpu(vmstat_uparam, cpu);
+
+ if (atomic_read(&vup->vmstat_work_enabled)) {
+ INIT_WORK(work, refresh_vm_stats);
+ schedule_work_on(cpu, work);
+ cpumask_set_cpu(cpu, &has_work);
+ }
+ }
+
+ for_each_cpu(cpu, &has_work)
+ flush_work(per_cpu_ptr(works, cpu));
+
+ put_online_cpus();
+ free_percpu(works);
+
for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
val = atomic_long_read(&vm_zone_stat[i]);
if (val < 0) {
@@ -1674,6 +1739,10 @@
/* Check processors whose vmstat worker threads have been disabled */
for_each_online_cpu(cpu) {
struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
+ struct vmstat_uparam *vup = &per_cpu(vmstat_uparam, cpu);
+
+ if (atomic_read(&vup->vmstat_work_enabled) == 0)
+ continue;
if (!delayed_work_pending(dw) && need_update(cpu))
queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
@@ -1696,6 +1765,135 @@
round_jiffies_relative(sysctl_stat_interval));
}
+#ifdef CONFIG_SYSFS
+
+static ssize_t vmstat_worker_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ unsigned int cpu = dev->id;
+ struct vmstat_uparam *vup = &per_cpu(vmstat_uparam, cpu);
+
+ return sprintf(buf, "%d\n", atomic_read(&vup->vmstat_work_enabled));
+}
+
+static ssize_t vmstat_worker_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ int ret, val;
+ struct vmstat_uparam *vup;
+ unsigned int cpu = dev->id;
+
+ ret = sscanf(buf, "%d", &val);
+ if (ret != 1 || val > 1 || val < 0)
+ return -EINVAL;
+
+ preempt_disable();
+
+ if (cpu_online(cpu)) {
+ vup = &per_cpu(vmstat_uparam, cpu);
+ atomic_set(&vup->vmstat_work_enabled, val);
+ } else
+ count = -EINVAL;
+
+ preempt_enable();
+
+ return count;
+}
+
+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_worker_attr =
+ __ATTR(vmstat_worker, 0644, vmstat_worker_show, vmstat_worker_store);
+
+struct device_attribute vmstat_threshold_attr =
+ __ATTR(vmstat_threshold, 0644, vmstat_thresh_show, vmstat_thresh_store);
+
+static struct attribute *vmstat_attrs[] = {
+ &vmstat_worker_attr.attr,
+ &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);
+ atomic_set(&vup->vmstat_work_enabled, 1);
+ }
+}
+
+#endif /* CONFIG_SYSFS */
+
static void __init init_cpu_node_state(void)
{
int node;
@@ -1723,9 +1921,13 @@
{
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);
+ atomic_set(&vup->vmstat_work_enabled, 1);
+
refresh_zone_stat_thresholds();
node_cpus = cpumask_of_node(node);
if (cpumask_weight(node_cpus) > 0)
@@ -1735,7 +1937,7 @@
return 0;
}
-#endif
+#endif /* CONFIG_SMP */
struct workqueue_struct *mm_percpu_wq;
@@ -1772,6 +1974,24 @@
#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 2017-04-25 08:46:25.237395070 -0300
@@ -0,0 +1,38 @@
+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.
+
+Usage example:
+=============
+
+To disable vmstat_update worker for cpu1:
+
+cd /sys/devices/system/cpu/cpu0/vmstat/
+
+# echo 1 > vmstat_threshold
+# echo 0 > vmstat_worker
+
+Setting vmstat_threshold to 1 means the per-CPU
+vmstat statistics will not be out-of-date
+for CPU 1.
+
+
--
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