Re: [PATCH v4 1/5] nohz_full: add support for "cpu_isolated" mode

On Wed, 22 Jul 2015, Paul E. McKenney wrote:

quoted

quoted

Didn't Christoph Lameter fix this?  Or is this an additional problem?

Well the vmstat update must realize first that there are no outstanding
updates before switching itself off. So typically there is one extra tick.
But we could add another function that will simply fold the differential
immediately and turn the kworker task in the expectation that the
processor will stay quiet.

Got it, thank you!

								Thanx, Paul

Ok here is a function that quiets down the vmstat kworkers.


Subject: vmstat: provide a function to quiet down the diff processing

quiet_vmstat() can be called in anticipation of a OS "quiet" period
where no tick processing should be triggered. quiet_vmstat() will fold
all pending differentials into the global counters and disable the
vmstat_worker processing.

Note that the shepherd thread will continue scanning the differentials
from another processor and will reenable the vmstat workers if it
detects any changes.

Signed-off-by: Christoph Lameter <redacted>

Index: linux/mm/vmstat.c
===================================================================

--- linux.orig/mm/vmstat.c
+++ linux/mm/vmstat.c

@@ -1394,6 +1394,20 @@ static void vmstat_update(struct work_st
 }

 /*
+ * Switch off vmstat processing and then fold all the remaining differentials
+ * until the diffs stay at zero. The function is used by NOHZ and can only be
+ * invoked when tick processing is not active.
+ */
+void quiet_vmstat(void)
+{
+	do {
+		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
+			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
+
+	} while (refresh_cpu_vm_stats());
+}
+
+/*
  * Check if the diffs for a certain cpu indicate that
  * an update is needed.
  */

Index: linux/include/linux/vmstat.h
===================================================================

--- linux.orig/include/linux/vmstat.h
+++ linux/include/linux/vmstat.h

@@ -211,6 +211,7 @@ extern void __inc_zone_state(struct zone
 extern void dec_zone_state(struct zone *, enum zone_stat_item);
 extern void __dec_zone_state(struct zone *, enum zone_stat_item);

+void quiet_vmstat(void);
 void cpu_vm_stats_fold(int cpu);
 void refresh_zone_stat_thresholds(void);

@@ -272,6 +273,7 @@ static inline void __dec_zone_page_state
 static inline void refresh_cpu_vm_stats(int cpu) { }
 static inline void refresh_zone_stat_thresholds(void) { }
 static inline void cpu_vm_stats_fold(int cpu) { }
+static inline void quiet_vmstat(void) { }

 static inline void drain_zonestat(struct zone *zone,
 			struct per_cpu_pageset *pset) { }