[PATCH v7 11/13] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely

Marcelo Tosatti
2023-03-20T18:21:01Z

With a task that busy loops on a given CPU,
the kworker interruption to execute vmstat_update
is undesired and may exceed latency thresholds
for certain applications.

Performance details for the kworker interruption:

oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ...
oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...

The example above shows an additional 7us for the

oslat -> kworker -> oslat

switches. In the case of a virtualized CPU, and the vmstat_update
interruption in the host (of a qemu-kvm vcpu), the latency penalty
observed in the guest is higher than 50us, violating the acceptable
latency threshold for certain applications.

To fix this, now that the counters are modified via cmpxchg
both CPU locally (via the account functions), and remotely (via
cpu_vm_stats_fold), its possible to switch vmstat_shepherd to perform
the per-CPU vmstats folding remotely.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---

Index: linux-vmstat-remote/mm/vmstat.c
===================================================================
--- linux-vmstat-remote.orig/mm/vmstat.c
+++ linux-vmstat-remote/mm/vmstat.c
@@ -2043,6 +2043,23 @@ static void vmstat_shepherd(struct work_

static DECLARE_DEFERRABLE_WORK(shepherd, vmstat_shepherd);

+#ifdef CONFIG_HAVE_CMPXCHG_LOCAL
+/* Flush counters remotely if CPU uses cmpxchg to update its per-CPU counters */
+static void vmstat_shepherd(struct work_struct *w)
+{
+ int cpu;
+
+ cpus_read_lock();
+ for_each_online_cpu(cpu) {
+ cpu_vm_stats_fold(cpu);
+ cond_resched();
+ }
+ cpus_read_unlock();
+
+ schedule_delayed_work(&shepherd,
+ round_jiffies_relative(sysctl_stat_interval));
+}
+#else
static void vmstat_shepherd(struct work_struct *w)
{
int cpu;
@@ -2062,6 +2079,7 @@ static void vmstat_shepherd(struct work_
schedule_delayed_work(&shepherd,
round_jiffies_relative(sysctl_stat_interval));
}
+#endif

static void __init start_shepherd_timer(void)
{