Both GNU and BSD makes share a "token pipe" between their instances, as
a result a -j $BIGNUM invocation results in multicore perf problems in
the poll handler.

Avoiding the store will reduce it a little bit. However, the crux of the
problem is the locked queuing up in poll_wait().

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---

disclaimer: could not be bothered to bench this

 fs/pipe.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 9841648c9cf3..e37c79935ecb 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -664,7 +664,8 @@ pipe_poll(struct file *filp, poll_table *wait)
 	union pipe_index idx;
 
 	/* Epoll has some historical nasty semantics, this enables them */
-	WRITE_ONCE(pipe->poll_usage, true);
+	if (unlikely(!READ_ONCE(pipe->poll_usage)))
+		WRITE_ONCE(pipe->poll_usage, true);
 
 	/*
 	 * Reading pipe state only -- no need for acquiring the semaphore.
-- 
2.48.1

On Sat 16-05-26 04:18:52, Mateusz Guzik wrote:
> Both GNU and BSD makes share a "token pipe" between their instances, as
> a result a -j $BIGNUM invocation results in multicore perf problems in
> the poll handler.
> 
> Avoiding the store will reduce it a little bit. However, the crux of the
> problem is the locked queuing up in poll_wait().
> 
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>

I guess doesn't harm to reduce cacheline invalidations. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> 
> disclaimer: could not be bothered to bench this
> 
>  fs/pipe.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 9841648c9cf3..e37c79935ecb 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -664,7 +664,8 @@ pipe_poll(struct file *filp, poll_table *wait)
>  	union pipe_index idx;
>  
>  	/* Epoll has some historical nasty semantics, this enables them */
> -	WRITE_ONCE(pipe->poll_usage, true);
> +	if (unlikely(!READ_ONCE(pipe->poll_usage)))
> +		WRITE_ONCE(pipe->poll_usage, true);
>  
>  	/*
>  	 * Reading pipe state only -- no need for acquiring the semaphore.
> -- 
> 2.48.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

On Sat, 16 May 2026 04:18:52 +0200, Mateusz Guzik wrote:
> Both GNU and BSD makes share a "token pipe" between their instances, as
> a result a -j $BIGNUM invocation results in multicore perf problems in
> the poll handler.
> 
> Avoiding the store will reduce it a little bit. However, the crux of the
> problem is the locked queuing up in poll_wait().
> 
> [...]

Applied to the vfs-7.2.misc branch of the vfs/vfs.git tree.
Patches in the vfs-7.2.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-7.2.misc

[1/1] fs/pipe: write to ->poll_usage only once
      https://git.kernel.org/vfs/vfs/c/e125daf61ff6