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

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On 23/10/2019 23.18, David Howells wrote:
Only do a wakeup in pipe_read() if we made space in a completely full
buffer.  The producer shouldn't be waiting on pipe->wait otherwise.

We could go further and wakeup writer only when at least half of buffer is empty.
This gives better batching and reduces rate of context switches.

https://lore.kernel.org/lkml/157219118016.7078.16223055699799396042.stgit@buzz/T/#u


Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

  fs/pipe.c |   15 ++++++---------
  1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 1274305772fb..e3a8f10750c9 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -327,11 +327,13 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
  				spin_lock_irq(&pipe->wait.lock);
  				tail++;
  				pipe_commit_read(pipe, tail);
-				do_wakeup = 0;
-				wake_up_interruptible_sync_poll_locked(
-					&pipe->wait, EPOLLOUT | EPOLLWRNORM);
+				do_wakeup = 1;
+				if (head - (tail - 1) == pipe->max_usage)
+					wake_up_interruptible_sync_poll_locked(
+						&pipe->wait, EPOLLOUT | EPOLLWRNORM);
  				spin_unlock_irq(&pipe->wait.lock);
-				kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
+				if (head - (tail - 1) == pipe->max_usage)
+					kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
  			}
  			total_len -= chars;
  			if (!total_len)
@@ -360,11 +362,6 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
  				ret = -ERESTARTSYS;
  			break;
  		}
-		if (do_wakeup) {
-			wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
- 			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-			do_wakeup = 0;
-		}
  		pipe_wait(pipe);
  	}
  	__pipe_unlock(pipe);






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