linux-kernel - [PATCH] select: do_pollfd: add unlikely branch hint return path

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Message-ID: <20250409143138.568173-1-colin.i.king@gmail.com>
Date: Wed,  9 Apr 2025 15:31:38 +0100
From: Colin Ian King <colin.i.king@...il.com>
To: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org
Cc: kernel-janitors@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] select: do_pollfd: add unlikely branch hint return path

Adding an unlikely() hint on the fd < 0 comparison return path improves
run-time performance of the mincore system call. gcov based coverage
analysis 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@...il.com>
---
 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