Micro optimise to get this_cpu once

[PATCH 2/4] sched/core: get this cpu once in ttwu_queue_cond

Posted by Shrikanth Hegde 1 week, 3 days ago

Calling smp_processor_id() on:
- In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
not print any warning.
- In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
__smp_processor_id

So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
it is better to cache the value. It could save a few cycles. Though
tiny, repeated could add up to a small value.

ttwu_queue_cond is called with interrupt disabled. So preemption is
disabled. Hence cache the value once instead.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 49af425f6a73..14996c7a0a31 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3900,6 +3900,8 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
 
 static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 {
+	int this_cpu = smp_processor_id();
+
 	/* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
 	if (!scx_allow_ttwu_queue(p))
 		return false;
@@ -3924,10 +3926,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 	 * If the CPU does not share cache, then queue the task on the
 	 * remote rqs wakelist to avoid accessing remote data.
 	 */
-	if (!cpus_share_cache(smp_processor_id(), cpu))
+	if (!cpus_share_cache(this_cpu, cpu))
 		return true;
 
-	if (cpu == smp_processor_id())
+	if (cpu == this_cpu)
 		return false;
 
 	/*
-- 
2.47.3

Re: [PATCH 2/4] sched/core: get this cpu once in ttwu_queue_cond

Posted by Mukesh Kumar Chaurasiya 1 week, 3 days ago

On Tue, Mar 24, 2026 at 01:06:28AM +0530, Shrikanth Hegde wrote:
> Calling smp_processor_id() on:
> - In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
> not print any warning.
> - In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
> __smp_processor_id
> 
> So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
> it is better to cache the value. It could save a few cycles. Though
> tiny, repeated could add up to a small value.
> 
> ttwu_queue_cond is called with interrupt disabled. So preemption is
> disabled. Hence cache the value once instead.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  kernel/sched/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 49af425f6a73..14996c7a0a31 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3900,6 +3900,8 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
>  
>  static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
>  {
> +	int this_cpu = smp_processor_id();
> +
>  	/* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
>  	if (!scx_allow_ttwu_queue(p))
>  		return false;
> @@ -3924,10 +3926,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
>  	 * If the CPU does not share cache, then queue the task on the
>  	 * remote rqs wakelist to avoid accessing remote data.
>  	 */
> -	if (!cpus_share_cache(smp_processor_id(), cpu))
> +	if (!cpus_share_cache(this_cpu, cpu))
>  		return true;
>  
> -	if (cpu == smp_processor_id())
> +	if (cpu == this_cpu)
>  		return false;
>  
>  	/*
> -- 
> 2.47.3
> 
LGTM

Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>

[tip: sched/core] sched/core: Get this cpu once in ttwu_queue_cond()

Posted by tip-bot2 for Shrikanth Hegde 1 week, 3 days ago

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

Commit-ID:     0e81fe79fec5a639700f09f39c8ab680c3312ba2
Gitweb:        https://git.kernel.org/tip/0e81fe79fec5a639700f09f39c8ab680c3312ba2
Author:        Shrikanth Hegde <sshegde@linux.ibm.com>
AuthorDate:    Tue, 24 Mar 2026 01:06:28 +05:30
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 24 Mar 2026 10:07:05 +01:00

sched/core: Get this cpu once in ttwu_queue_cond()

Calling smp_processor_id() on:
 - In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
   not print any warning.
 - In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
   __smp_processor_id

So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
it is better to cache the value. It could save a few cycles. Though
tiny, repeated could add up to a small value.

ttwu_queue_cond is called with interrupt disabled. So preemption is
disabled. Hence cache the value once instead.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Link: https://patch.msgid.link/20260323193630.640311-3-sshegde@linux.ibm.com
---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 64b467c..7c7d4bf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3842,6 +3842,8 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
 
 static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 {
+	int this_cpu = smp_processor_id();
+
 	/* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
 	if (!scx_allow_ttwu_queue(p))
 		return false;
@@ -3866,10 +3868,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 	 * If the CPU does not share cache, then queue the task on the
 	 * remote rqs wakelist to avoid accessing remote data.
 	 */
-	if (!cpus_share_cache(smp_processor_id(), cpu))
+	if (!cpus_share_cache(this_cpu, cpu))
 		return true;
 
-	if (cpu == smp_processor_id())
+	if (cpu == this_cpu)
 		return false;
 
 	/*