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

Marcelo Tosatti
2023-02-01T19:53:29Z

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)
{