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>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5176d51..5ecf498 100644 (file)
--- 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);