pipe: Increase the writer-wakeup threshold to reduce context-switch count

Increase the threshold at which the reader sends a wake event to the
writers in the queue such that the queue must be half empty before the wake
is issued rather than the wake being issued when just a single slot
available.

This reduces the number of context switches in the tests significantly,
without altering the amount of work achieved.  With my pipe-bench program,
there's a 20% reduction versus an unpatched kernel.

Suggested-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/fs/pipe.c b/fs/pipe.c
index aba2455..9cd5cbe 100644 (file)
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -324,16 +324,18 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
                        }
 
                        if (!buf->len) {
+                               bool wake;
                                pipe_buf_release(pipe, buf);
                                spin_lock_irq(&pipe->wait.lock);
                                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;