[PATCH][V2] select: do_pollfd: add unlikely branch hint return path

[PATCH][V2] select: do_pollfd: add unlikely branch hint return path

[Colin Ian King]

Adding an unlikely() hint on the fd < 0 comparison return path improves
run-time performance of the poll() system call. gcov based coverage
analysis based on running stress-ng and a kernel build shows that this
path return path is highly unlikely.

Benchmarking on an Debian based Intel(R) Core(TM) Ultra 9 285K with
a 6.15-rc1 kernel and a poll of 1024 file descriptors with zero timeout
shows an call reduction from 32818 ns down to 32635 ns, which is a ~0.5%
performance improvement.

Results based on running 25 tests with turbo disabled (to reduce clock
freq turbo changes), with 30 second run per test and comparing the number
of poll() calls per second. The % standard deviation of the 25 tests
was 0.08%, so results are reliable.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
---

V2: replace mincore with poll() in commit message to fix a cut-n-paste error,
    add more info about the gcov analysis

---
 fs/select.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/select.c b/fs/select.c
index 7da531b1cf6b..0eaf3522abe9 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -857,7 +857,7 @@ static inline __poll_t do_pollfd(struct pollfd *pollfd, poll_table *pwait,
 	int fd = pollfd->fd;
 	__poll_t mask, filter;
 
-	if (fd < 0)
+	if (unlikely(fd < 0))
 		return 0;
 
 	CLASS(fd, f)(fd);
-- 
2.49.0

Re: [PATCH][V2] select: do_pollfd: add unlikely branch hint return path

[Christian Brauner]

On Wed, 09 Apr 2025 16:55:10 +0100, Colin Ian King wrote:
> Adding an unlikely() hint on the fd < 0 comparison return path improves
> run-time performance of the poll() system call. gcov based coverage
> analysis based on running stress-ng and a kernel build shows that this
> path return path is highly unlikely.
> 
> Benchmarking on an Debian based Intel(R) Core(TM) Ultra 9 285K with
> a 6.15-rc1 kernel and a poll of 1024 file descriptors with zero timeout
> shows an call reduction from 32818 ns down to 32635 ns, which is a ~0.5%
> performance improvement.
> 
> [...]

Applied to the vfs-6.16.misc branch of the vfs/vfs.git tree.
Patches in the vfs-6.16.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-6.16.misc

[1/1] select: do_pollfd: add unlikely branch hint return path
      https://git.kernel.org/vfs/vfs/c/5730609ffd7e