[PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush unmapped pages

[Mel Gorman <mgorman@suse.de>]

The patch "mm: Send a single IPI to TLB flush multiple pages when unmapping"
would batch 32 pages before sending an IPI. This patch increases the size of
the data structure to hold a pages worth of PFNs before sending an IPI. This
is a trade-off between memory usage and reducing IPIS sent. In the ideal
case where multiple processes are reading large mapped files, this patch
reduces interrupts/second from roughly 180K per second to 60K per second.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 include/linux/sched.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9d51841806f4..abff66ecc302 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1275,11 +1275,16 @@ enum perf_event_task_context {
 	perf_nr_task_contexts,
 };
 
-/* Matches SWAP_CLUSTER_MAX but refined to limit header dependencies */
-#define BATCH_TLBFLUSH_SIZE 32UL
+/*
+ * Use a page to store as many PFNs as possible for batch unmapping. Adjusting
+ * this trades memory usage for number of IPIs sent
+ */
+#define BATCH_TLBFLUSH_SIZE \
+	((PAGE_SIZE - sizeof(struct cpumask) - sizeof(unsigned long)) / sizeof(unsigned long))
 
 /* Track pages that require TLB flushes */
 struct unmap_batch {
+	/* Update BATCH_TLBFLUSH_SIZE when adjusting this structure */
 	struct cpumask cpumask;
 	unsigned long nr_pages;
 	unsigned long pfns[BATCH_TLBFLUSH_SIZE];
-- 
2.1.2