flush_cpu_workqueue: don't flush an empty ->worklist

Now when we have ->current_work we can avoid adding a barrier and waiting
for its completition when cwq's queue is empty.

Note: this change is also useful if we change flush_workqueue() to also
check the dead CPUs.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Gautham Shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 8 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5176d51bcc2a..5ecf4984e382 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -404,12 +404,15 @@ static void wq_barrier_func(struct work_struct *work)
         complete(&barr->done);
 }
 
-static inline void init_wq_barrier(struct wq_barrier *barr)
+static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
+                                        struct wq_barrier *barr, int tail)
 {
         INIT_WORK(&barr->work, wq_barrier_func);
         __set_bit(WORK_STRUCT_PENDING, work_data_bits(&barr->work));
 
         init_completion(&barr->done);
+
+        insert_work(cwq, &barr->work, tail);
 }
 
 static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
@@ -428,13 +431,20 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
                 preempt_disable();
         } else {
                 struct wq_barrier barr;
+                int active = 0;
 
-                init_wq_barrier(&barr);
-                __queue_work(cwq, &barr.work);
+                spin_lock_irq(&cwq->lock);
+                if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
+                        insert_wq_barrier(cwq, &barr, 1);
+                        active = 1;
+                }
+                spin_unlock_irq(&cwq->lock);
 
-                preempt_enable();       /* Can no longer touch *cwq */
-                wait_for_completion(&barr.done);
-                preempt_disable();
+                if (active) {
+                        preempt_enable();
+                        wait_for_completion(&barr.done);
+                        preempt_disable();
+                }
         }
 }
 
@@ -475,8 +485,7 @@ static void wait_on_work(struct cpu_workqueue_struct *cwq,
 
         spin_lock_irq(&cwq->lock);
         if (unlikely(cwq->current_work == work)) {
-                init_wq_barrier(&barr);
-                insert_work(cwq, &barr.work, 0);
+                insert_wq_barrier(cwq, &barr, 0);
                 running = 1;
         }
         spin_unlock_irq(&cwq->lock);