[PATCH] timers/migration: remove an unneeded call to find_first_bit()

[PATCH] timers/migration: remove an unneeded call to find_first_bit()

[Petr Tesarik]

Use simple bit arithmetic to extract the least significant bit from the
active bitmask, because the bit position is not needed, only the bit
itself.

No functional change.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
---
 kernel/time/timer_migration.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 2f6330831f08..0f40727b6cac 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1277,12 +1277,12 @@ static bool tmigr_inactive_up(struct tmigr_group *group,
 			 * group is idle!
 			 */
 			if (!childstate.active) {
-				unsigned long new_migr_bit, active = newstate.active;
+				u8 active = newstate.active;
 
-				new_migr_bit = find_first_bit(&active, BIT_CNT);
+				if (active) {
+					u8 lsbit = active & -active;
 
-				if (new_migr_bit != BIT_CNT) {
-					newstate.migrator = BIT(new_migr_bit);
+					newstate.migrator = lsbit;
 				} else {
 					newstate.migrator = TMIGR_NONE;
 
-- 
2.49.0

Re: [PATCH] timers/migration: remove an unneeded call to find_first_bit()

[Petr Tesarik]

On Fri, 30 May 2025 15:25:39 +0200
Petr Tesarik <ptesarik@suse.com> wrote:

> Use simple bit arithmetic to extract the least significant bit from the
> active bitmask, because the bit position is not needed, only the bit
> itself.

There has been no reply here, but I received an out-of-band
recommendation to wrap this bit-twiddling trick in a macro.

So, you can ignore this patch for now and wait for a respin after I
add the macro and convert pre-existing uses in the kernel.

Petr T

> No functional change.
> 
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>
> ---
>  kernel/time/timer_migration.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
> index 2f6330831f08..0f40727b6cac 100644
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -1277,12 +1277,12 @@ static bool tmigr_inactive_up(struct tmigr_group *group,
>  			 * group is idle!
>  			 */
>  			if (!childstate.active) {
> -				unsigned long new_migr_bit, active = newstate.active;
> +				u8 active = newstate.active;
>  
> -				new_migr_bit = find_first_bit(&active, BIT_CNT);
> +				if (active) {
> +					u8 lsbit = active & -active;
>  
> -				if (new_migr_bit != BIT_CNT) {
> -					newstate.migrator = BIT(new_migr_bit);
> +					newstate.migrator = lsbit;
>  				} else {
>  					newstate.migrator = TMIGR_NONE;
>