[PATCH 4/5] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely

[Marcelo Tosatti <mtosatti@redhat.com>]

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.

This fixes the following two problems:

 1. A customer provided some evidence which indicates that
    the idle tick was stopped; albeit, CPU-specific vmstat
    counters still remained populated.

    Thus one can only assume quiet_vmstat() was not
    invoked on return to the idle loop. If I understand
    correctly, I suspect this divergence might erroneously
    prevent a reclaim attempt by kswapd. If the number of
    zone specific free pages are below their per-cpu drift
    value then zone_page_state_snapshot() is used to
    compute a more accurate view of the aforementioned
    statistic.  Thus any task blocked on the NUMA node
    specific pfmemalloc_wait queue will be unable to make
    significant progress via direct reclaim unless it is
    killed after being woken up by kswapd
    (see throttle_direct_reclaim())

 2. With a SCHED_FIFO task that busy loops on a given CPU,
    and kworker for that CPU at SCHED_OTHER priority,
    queuing work to sync per-vmstats will either cause that
    work to never execute, or stalld (i.e. stall daemon)
    boosts kworker priority which causes a latency
    violation

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
@@ -2007,6 +2007,23 @@ 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;
@@ -2026,6 +2043,7 @@ static void vmstat_shepherd(struct work_
 	schedule_delayed_work(&shepherd,
 		round_jiffies_relative(sysctl_stat_interval));
 }
+#endif
 
 static void __init start_shepherd_timer(void)
 {