Re: [PATCH v4] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 11/12/25 4:07 PM, Peter Zijlstra wrote:
> On Wed, Nov 12, 2025 at 01:32:23PM +0530, Srikar Dronamraju wrote:
>>>   	group = sched_balance_find_src_group(&env);
>>>   	if (!group) {
>>>   		schedstat_inc(sd->lb_nobusyg[idle]);
>>> @@ -11892,6 +11916,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>>>   			if (!cpumask_subset(cpus, env.dst_grpmask)) {
>>>   				env.loop = 0;
>>>   				env.loop_break = SCHED_NR_MIGRATE_BREAK;
>>> +				if (need_unlock)
>>> +					atomic_set_release(&sched_balance_running, 0);
>>> +
>>
>> One nit:
>> While the current code is good, would conditionally resetting the
>> need_unlock just after resetting the atomic variable better than
>> unconditional reset that we do now?
> 
> Right, I had the same thought when grabbed the patch yesterday, but
> ignored it.
> 
> But perhaps something like so?
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11717,14 +11717,20 @@ static int sched_balance_rq(int this_cpu
>   		.fbq_type	= all,
>   		.tasks		= LIST_HEAD_INIT(env.tasks),
>   	};
> -	bool need_unlock;
> +	bool need_unlock = false;
>   
>   	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>   
>   	schedstat_inc(sd->lb_count[idle]);
>   
> +	if (0) {
>   redo:
> -	need_unlock = false;
> +		if (need_unlock) {
> +			atomic_set_release(&sched_balance_running, 0);
> +			need_unlock = false;
> +		}
> +	}
> +
>   	if (!should_we_balance(&env)) {
>   		*continue_balancing = 0;
>   		goto out_balanced;
> @@ -11861,9 +11867,6 @@ static int sched_balance_rq(int this_cpu
>   			if (!cpumask_subset(cpus, env.dst_grpmask)) {
>   				env.loop = 0;
>   				env.loop_break = SCHED_NR_MIGRATE_BREAK;
> -				if (need_unlock)
> -					atomic_set_release(&sched_balance_running, 0);
> -
>   				goto redo;
>   			}
>   			goto out_all_pinned;
> 
> 
> ---
> 
> The other option is something like this, but Linus hated on this pattern
> when we started with things.
> 
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11692,6 +11692,8 @@ static void update_lb_imbalance_stat(str
>    */
>   static atomic_t sched_balance_running = ATOMIC_INIT(0);
>   
> +DEFINE_FREE(balance_unlock, atomic_t *, if (_T) atomic_set_release(_T, 0));
> +
>   /*
>    * Check this_cpu to ensure it is balanced within domain. Attempt to move
>    * tasks if there is an imbalance.
> @@ -11717,24 +11719,23 @@ static int sched_balance_rq(int this_cpu
>   		.fbq_type	= all,
>   		.tasks		= LIST_HEAD_INIT(env.tasks),
>   	};
> -	bool need_unlock;
>   
>   	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>   
>   	schedstat_inc(sd->lb_count[idle]);
>   
>   redo:
> -	need_unlock = false;
>   	if (!should_we_balance(&env)) {
>   		*continue_balancing = 0;
> -		goto out_balanced;
> +		return 0;
>   	}
>   
> +	atomic_t *lock __free(balance_unlock) = NULL;
>   	if (sd->flags & SD_SERIALIZE) {
> -		if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) {
> -			goto out_balanced;
> -		}
> -		need_unlock = true;
> +		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
> +			return 0;
> +
> +		lock = &sched_balance_running;
>   	}
>   
>   	group = sched_balance_find_src_group(&env);
> @@ -11861,9 +11862,6 @@ static int sched_balance_rq(int this_cpu
>   			if (!cpumask_subset(cpus, env.dst_grpmask)) {
>   				env.loop = 0;
>   				env.loop_break = SCHED_NR_MIGRATE_BREAK;
> -				if (need_unlock)
> -					atomic_set_release(&sched_balance_running, 0);
> -
>   				goto redo;
>   			}
>   			goto out_all_pinned;
> @@ -11980,9 +11978,6 @@ static int sched_balance_rq(int this_cpu
>   	    sd->balance_interval < sd->max_interval)
>   		sd->balance_interval *= 2;
>   out:
> -	if (need_unlock)
> -		atomic_set_release(&sched_balance_running, 0);
> -
>   	return ld_moved;
>   }
>   

Second one is difficult to understand.


Is this an option too?
---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f8e1df9c5199..64e2aeacd65e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11720,14 +11720,13 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
                 .fbq_type       = all,
                 .tasks          = LIST_HEAD_INIT(env.tasks),
         };
-       bool need_unlock;
+       bool need_unlock = false;
  
         cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
  
         schedstat_inc(sd->lb_count[idle]);
  
  redo:
-       need_unlock = false;
         if (!should_we_balance(&env)) {
                 *continue_balancing = 0;
                 goto out_balanced;
@@ -11864,8 +11863,10 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
                         if (!cpumask_subset(cpus, env.dst_grpmask)) {
                                 env.loop = 0;
                                 env.loop_break = SCHED_NR_MIGRATE_BREAK;
-                               if (need_unlock)
+                               if (need_unlock) {
                                         atomic_set_release(&sched_balance_running, 0);
+                                       need_unlock = false;
+                               }
  
                                 goto redo;
                         }