[PATCH 3/3] mm,vmacache: optimize overflow system-wide flushing

[Davidlohr Bueso <davidlohr@hp.com>]

For single threaded workloads, we can avoid flushing
and iterating through the entire list of tasks, making
the whole function a lot faster, requiring only a single
atomic read for the mm_users.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
---
 mm/vmacache.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/vmacache.c b/mm/vmacache.c
index e167da2..61c38ae 100644
--- a/mm/vmacache.c
+++ b/mm/vmacache.c
@@ -17,6 +17,16 @@ void vmacache_flush_all(struct mm_struct *mm)
 {
 	struct task_struct *g, *p;
 
+	/*
+	 * Single threaded tasks need not iterate the entire
+	 * list of process. We can avoid the flushing as well
+	 * since the mm's seqnum was increased and don't have
+	 * to worry about other threads' seqnum. Current's
+	 * flush will occur upon the next lookup.
+	 */
+	if (atomic_read(&mm->mm_users) == 1)
+		return;
+
 	rcu_read_lock();
 	for_each_process_thread(g, p) {
 		/*
-- 
1.8.1.4

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