===================================================================
@@ -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)
{
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> ---