mm,vmacache: optimize overflow system-wide flushing

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.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/mm/vmacache.c b/mm/vmacache.c
index 658ed3b3e38d..9f25af825dec 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) {
                 /*