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

From: Linus Torvalds
Date: Tue Aug 24 2021 - 13:59:08 EST


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)) {