This patch introduces two new vmstat counters for testing purposes:
pcp_global_drain that counts the number of times a per-cpu pages
global drain was requested and pcp_global_ipi_saved that counts
the number of times the number of CPUs with per-cpu pages in any
zone were less then 1/2 of the number of online CPUs.

The patch purpose is to show the usefulness of only sending an IPI
asking to drain per-cpu pages to CPUs that actually have them
instead of a blind global IPI. It is not inteded to be merged.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Acked-by: Peter Zijlstra <redacted>
CC: Christoph Lameter <redacted>
CC: Chris Metcalf <redacted>
CC: Frederic Weisbecker <redacted>
CC: linux-mm@kvack.org
CC: Pekka Enberg <penberg@kernel.org>
CC: Matt Mackall <redacted>
CC: Sasha Levin <redacted>
CC: Rik van Riel <redacted>
CC: Andi Kleen <redacted>
CC: Mel Gorman <redacted>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Avi Kivity <redacted>
CC: Michal Nazarewicz <redacted>
CC: Kosaki Motohiro <redacted>
CC: Milton Miller <redacted>
---
 include/linux/vm_event_item.h |    1 +
 mm/page_alloc.c               |    5 +++++
 mm/vmstat.c                   |    2 ++
 3 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 03b90cd..3657f6f 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h

@@ -58,6 +58,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_COLLAPSE_ALLOC_FAILED,
 		THP_SPLIT,
 #endif
+		PCP_GLOBAL_DRAIN, PCP_GLOBAL_IPI_SAVED,
 		NR_VM_EVENT_ITEMS
 };
 

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3ff5aff..09d47eb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -1201,6 +1201,11 @@ void drain_all_pages(void)
 			cpumask_clear_cpu(cpu, &cpus_with_pcps);
 	}
 	on_each_cpu_mask(&cpus_with_pcps, drain_local_pages, NULL, 1);
+
+	count_vm_event(PCP_GLOBAL_DRAIN);
+	if (cpumask_weight(&cpus_with_pcps) <
+	   (cpumask_weight(cpu_online_mask) / 2))
+		count_vm_event(PCP_GLOBAL_IPI_SAVED);
 }
 
 #ifdef CONFIG_HIBERNATION

diff --git a/mm/vmstat.c b/mm/vmstat.c
index f600557..3ee5f99 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c

@@ -786,6 +786,8 @@ const char * const vmstat_text[] = {
 	"thp_collapse_alloc_failed",
 	"thp_split",
 #endif
+	"pcp_global_drain",
+	"pcp_global_ipi_saved"
 
 #endif /* CONFIG_VM_EVENTS_COUNTERS */
 };

-- 
1.7.0.4

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