[RFC PATCH 07/10] pipe: Conditionalise wakeup in pipe_read() [ver #2]

[David Howells]

Okay, attached is a change that might give you what you want.  I tried my
pipe-bench program (see cover note) with perf.  The output of the program with
the patch applied was:

-       pipe                  305127298     36262221772       302185181         7887690

The output of perf with the patch applied:

        239,943.92 msec task-clock                #    1.997 CPUs utilized
            17,728      context-switches          #   73.884 M/sec
               124      cpu-migrations            #    0.517 M/sec
             9,330      page-faults               #   38.884 M/sec
   885,107,207,365      cycles                    # 3688822.793 GHz
 1,386,873,499,490      instructions              #    1.57  insn per cycle
   311,037,372,339      branches                  # 1296296921.931 M/sec
        33,467,827      branch-misses             #    0.01% of all branches

And without:

        239,891.87 msec task-clock                #    1.997 CPUs utilized
            22,187      context-switches          #   92.488 M/sec
               133      cpu-migrations            #    0.554 M/sec
             9,334      page-faults               #   38.909 M/sec
   884,906,976,128      cycles                    # 3688787.725 GHz
 1,391,986,932,265      instructions              #    1.57  insn per cycle
   311,394,686,857      branches                  # 1298067400.849 M/sec
        30,242,823      branch-misses             #    0.01% of all branches

So it did make something like a 20% reduction in context switches.

David
---
diff --git a/fs/pipe.c b/fs/pipe.c
index e3d5f7a39123..5167921edd73 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -276,7 +276,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	size_t total_len = iov_iter_count(to);
 	struct file *filp = iocb->ki_filp;
 	struct pipe_inode_info *pipe = filp->private_data;
-	int do_wakeup;
+	int do_wakeup, wake;
 	ssize_t ret;

 	/* Null read succeeds. */
@@ -329,11 +329,12 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				tail++;
 				pipe->tail = tail;
 				do_wakeup = 1;
-				if (head - (tail - 1) == pipe->max_usage)
+				wake = head - (tail - 1) == pipe->max_usage / 2;
+				if (wake)
 					wake_up_interruptible_sync_poll_locked(
 						&pipe->wait, EPOLLOUT | EPOLLWRNORM);
 				spin_unlock_irq(&pipe->wait.lock);
-				if (head - (tail - 1) == pipe->max_usage)
+				if (wake)
 					kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 			}
 			total_len -= chars;