[PATCH] kcmp: improve performance adding an unlikely hint to task comparisons

[PATCH] kcmp: improve performance adding an unlikely hint to task comparisons

[Colin Ian King]

Adding an unlikely() hint on task comparisons on an unlikely error
return path improves run-time performance of the kcmp system call.

Benchmarking on an i9-12900 shows an improvement of ~5.5% on kcmp().
Results based on running 20 tests with turbo disabled (to reduce
clock freq turbo changes), with 10 second run per test and comparing
the number of kcmp calls per second. The % Standard deviation of 20
tests was ~0.25%, results are reliable.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
---
 kernel/kcmp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index 2c596851f8a9..7c1a65bd5f8d 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -145,7 +145,7 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 	 */
 	task1 = find_task_by_vpid(pid1);
 	task2 = find_task_by_vpid(pid2);
-	if (!task1 || !task2)
+	if (unlikely(!task1 || !task2))
 		goto err_no_task;
 
 	get_task_struct(task1);
-- 
2.47.2

Re: [PATCH] kcmp: improve performance adding an unlikely hint to task comparisons

[Christian Brauner]

On Thu, 13 Feb 2025 16:39:16 +0000, Colin Ian King wrote:
> Adding an unlikely() hint on task comparisons on an unlikely error
> return path improves run-time performance of the kcmp system call.
> 
> Benchmarking on an i9-12900 shows an improvement of ~5.5% on kcmp().
> Results based on running 20 tests with turbo disabled (to reduce
> clock freq turbo changes), with 10 second run per test and comparing
> the number of kcmp calls per second. The % Standard deviation of 20
> tests was ~0.25%, results are reliable.
> 
> [...]

Applied to the vfs-6.15.misc branch of the vfs/vfs.git tree.
Patches in the vfs-6.15.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-6.15.misc

[1/1] kcmp: improve performance adding an unlikely hint to task comparisons
      https://git.kernel.org/vfs/vfs/c/a25b006052a7