Re: [pipe] 3b844826b6: stress-ng.sigio.ops_per_sec -99.3% regression

[Linus Torvalds]

On Tue, Aug 24, 2021 at 10:32 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> We could do the same ugly thing for FASYNC that we do for EPOLLET -
> make it always fasync on new data, exactly because the previous SIGIO
> might not have emptied the buffer completely.

The patch would be something like the attached (UNTESTED!)

                  Linus
 fs/pipe.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 678dee2a8228..6d4342bad9f1 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -363,10 +363,9 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		 * _very_ unlikely case that the pipe was full, but we got
 		 * no data.
 		 */
-		if (unlikely(was_full)) {
+		if (unlikely(was_full))
 			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
-			kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-		}
+		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 
 		/*
 		 * But because we didn't read anything, at this point we can
@@ -385,12 +384,11 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		wake_next_reader = false;
 	__pipe_unlock(pipe);
 
-	if (was_full) {
+	if (was_full)
 		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
-		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-	}
 	if (wake_next_reader)
 		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
+	kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 	if (ret > 0)
 		file_accessed(filp);
 	return ret;
@@ -565,10 +563,9 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		 * become empty while we dropped the lock.
 		 */
 		__pipe_unlock(pipe);
-		if (was_empty) {
+		if (was_empty)
 			wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
-			kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
-		}
+		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		wait_event_interruptible_exclusive(pipe->wr_wait, pipe_writable(pipe));
 		__pipe_lock(pipe);
 		was_empty = pipe_empty(pipe->head, pipe->tail);
@@ -591,10 +588,9 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	 * Epoll nonsensically wants a wakeup whether the pipe
 	 * was already empty or not.
 	 */
-	if (was_empty || pipe->poll_usage) {
+	if (was_empty || pipe->poll_usage)
 		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
-		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
-	}
+	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	if (wake_next_writer)
 		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
 	if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) {