[PATCH 6/6] mm: Gather more PFNs before sending a TLB to flush unmapped pages

[Mel Gorman <mgorman@xxxxxxx>]

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@xxxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
---
 include/linux/sched.h | 9 +++++----
 kernel/fork.c         | 6 ++++--
 mm/vmscan.c           | 5 +++--
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5c09db02fe78..3e4d3f545005 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1275,16 +1275,17 @@ enum perf_event_task_context {
 	perf_nr_task_contexts,
 };
 
-/* Matches SWAP_CLUSTER_MAX but refined to limit header dependencies */
-#define BATCH_TLBFLUSH_SIZE 32UL
-
 /* Track pages that require TLB flushes */
 struct tlbflush_unmap_batch {
 	struct cpumask cpumask;
 	unsigned long nr_pages;
-	unsigned long pfns[BATCH_TLBFLUSH_SIZE];
+	unsigned long pfns[0];
 };
 
+/* alloc_tlb_ubc() always allocates a page */
+#define BATCH_TLBFLUSH_SIZE \
+	((PAGE_SIZE - sizeof(struct tlbflush_unmap_batch)) / sizeof(unsigned long))
+
 struct task_struct {
 	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
 	void *stack;
diff --git a/kernel/fork.c b/kernel/fork.c
index 86c872fec9fb..f260663f209a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -247,8 +247,10 @@ void __put_task_struct(struct task_struct *tsk)
 	put_signal_struct(tsk->signal);
 
 #ifdef CONFIG_ARCH_SUPPORTS_LOCAL_TLB_PFN_FLUSH
-	kfree(tsk->tlb_ubc);
-	tsk->tlb_ubc = NULL;
+	if (tsk->tlb_ubc) {
+		free_page((unsigned long)tsk->tlb_ubc);
+		tsk->tlb_ubc = NULL;
+	}
 #endif
 
 	if (!profile_handoff_task(tsk))
diff --git a/mm/vmscan.c b/mm/vmscan.c
index e39e7c4bf548..080ba929049c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2775,14 +2775,15 @@ out:
 /*
  * Allocate the control structure for batch TLB flushing. An allocation
  * failure is harmless as the reclaimer will send IPIs where necessary.
+ * If the allocation size changes then update BATCH_TLBFLUSH_SIZE.
  */
 void alloc_tlb_ubc(void)
 {
 	if (current->tlb_ubc)
 		return;
 
-	current->tlb_ubc = kmalloc(sizeof(struct tlbflush_unmap_batch),
-						GFP_ATOMIC | __GFP_NOWARN);
+	current->tlb_ubc = (struct tlbflush_unmap_batch *)
+				__get_free_page(GFP_KERNEL | __GFP_NOWARN);
 	if (!current->tlb_ubc)
 		return;
 
-- 
2.1.2

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