[PATCH v3 3/3] mm/vmstat: do not refresh stats for isolated CPUs

[Marcelo Tosatti <mtosatti@redhat.com>]

schedule_work_on API uses the workqueue mechanism to
queue a work item on a queue. A kernel thread, which
runs on the target CPU, executes those work items.

Therefore, when using the schedule_work_on API,
it is necessary for the kworker kernel thread to
be scheduled in, for the work function to be executed.

Time sensitive applications such as SoftPLCs
(https://tum-esi.github.io/publications-list/PDF/2022-ETFA-How_Real_Time_Are_Virtual_PLCs.pdf),
have their response times affected by such interruptions.

The /proc/sys/vm/stat_refresh file was originally introduced
with the goal to:

"Provide /proc/sys/vm/stat_refresh to force an immediate update of
 per-cpu into global vmstats: useful to avoid a sleep(2) or whatever
 before checking counts when testing.  Originally added to work around a
 bug which left counts stranded indefinitely on a cpu going idle (an
 inaccuracy magnified when small below-batch numbers represent "huge"
 amounts of memory), but I believe that bug is now fixed: nonetheless,
 this is still a useful knob."

Other than the potential interruption to a time sensitive application,
if using SCHED_FIFO or SCHED_RR priority on the isolated CPU, then
system hangs can occur:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978688

To avoid the problems above, do not schedule the work to synchronize
per-CPU mm counters on isolated CPUs. Given the possibility for
breaking existing userspace applications, avoid returning
errors from access to /proc/sys/vm/stat_refresh.

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

---
v3: improve changelog		  (Michal Hocko)
v2: opencode schedule_on_each_cpu (Michal Hocko)

Index: linux-vmstat-remote/mm/vmstat.c
===================================================================
--- linux-vmstat-remote.orig/mm/vmstat.c
+++ linux-vmstat-remote/mm/vmstat.c
@@ -1881,8 +1881,13 @@ int vmstat_refresh(struct ctl_table *tab
 		   void *buffer, size_t *lenp, loff_t *ppos)
 {
 	long val;
-	int err;
 	int i;
+	int cpu;
+	struct work_struct __percpu *works;
+
+	works = alloc_percpu(struct work_struct);
+	if (!works)
+		return -ENOMEM;
 
 	/*
 	 * The regular update, every sysctl_stat_interval, may come later
@@ -1896,9 +1901,24 @@ int vmstat_refresh(struct ctl_table *tab
 	 * 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;
+	cpus_read_lock();
+	for_each_online_cpu(cpu) {
+		struct work_struct *work;
+
+		if (cpu_is_isolated(cpu))
+			continue;
+		work = per_cpu_ptr(works, cpu);
+		INIT_WORK(work, refresh_vm_stats);
+		schedule_work_on(cpu, work);
+	}
+
+	for_each_online_cpu(cpu) {
+		if (cpu_is_isolated(cpu))
+			continue;
+		flush_work(per_cpu_ptr(works, cpu));
+	}
+	cpus_read_unlock();
+	free_percpu(works);
 	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
 		/*
 		 * Skip checking stats known to go negative occasionally.