[v1] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

[PATCH RESEND] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Yury Norov (NVIDIA) 2 months ago

cpumask_empty() call is O(N) and useless because the previous
cpumask_and() returns false for empty 'cpus'. Drop it.

CC: Markus Elfring <Markus.Elfring@web.de>
Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
Rebased and re-tested on top of master. Original patch:
https://lore.kernel.org/all/20250911023426.457745-1-yury.norov@gmail.com/

 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e04f94233cda..af9e9dbdaa6d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8362,9 +8362,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 		int max_spare_cap_cpu = -1;
 		int fits, max_fits = -1;
 
-		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
-
-		if (cpumask_empty(cpus))
+		if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
 			continue;
 
 		/* Account external pressure for the energy estimation */
-- 
2.43.0

Re: [PATCH RESEND] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Peter Zijlstra 2 months ago

On Sat, Dec 06, 2025 at 11:05:42PM -0500, Yury Norov (NVIDIA) wrote:
> cpumask_empty() call is O(N) and useless because the previous
> cpumask_and() returns false for empty 'cpus'. Drop it.
> 
> CC: Markus Elfring <Markus.Elfring@web.de>
> Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
> Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

Picked it (and the other two) up for processing after rc1.

[tip: sched/core] sched/fair: Drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by tip-bot2 for Yury Norov (NVIDIA) 3 weeks, 6 days ago

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     ff1de90dd7a69ef43586683535ad87ab899a1214
Gitweb:        https://git.kernel.org/tip/ff1de90dd7a69ef43586683535ad87ab899a1214
Author:        Yury Norov (NVIDIA) <yury.norov@gmail.com>
AuthorDate:    Sat, 06 Dec 2025 23:05:42 -05:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 08 Jan 2026 12:43:56 +01:00

sched/fair: Drop useless cpumask_empty() in find_energy_efficient_cpu()

cpumask_empty() call is O(N) and useless because the previous
cpumask_and() returns false for empty 'cpus'. Drop it.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20251207040543.407695-1-yury.norov@gmail.com
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7377f91..64275d7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8359,9 +8359,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 		int max_spare_cap_cpu = -1;
 		int fits, max_fits = -1;
 
-		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
-
-		if (cpumask_empty(cpus))
+		if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
 			continue;
 
 		/* Account external pressure for the energy estimation */