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

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

On 11/13/25 2:40 AM, Tim Chen wrote:
> On Wed, 2025-11-12 at 12:21 +0100, Peter Zijlstra wrote:
>> On Wed, Nov 12, 2025 at 04:39:43PM +0530, Shrikanth Hegde wrote:
>>>
>>>
>>
>>>> So perhaps this is the better option -- or did I overlook something with
>>>> should_we_balance? It doesn't look like that will make a different
>>>> decision on the retry.
>>>>
>>>
>>> I think in newidle balance, these checks are there in swb to bail of load balance.
>>> redo logic catches it right?
>>
>> Urgh, my brain still thinks we're not serializing on newidle. Perhaps I
>> should make this 2 patches, one moving the serializing and one adding it
>> to newidle.
>>
>>> env->dst_rq lock is taken only in attach_tasks, meanwhile, if the wakeup happened,
>>> pending would be set. is irq enabled or remote CPU can set ttwu_pending on this rq?
>>>
>>>          if (env->idle == CPU_NEWLY_IDLE) {
>>>                  if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
>>>                          return 0;
>>>                  return 1;
>>>          }
>>
>> Right, that could get tickled.
> 
> How about something like the following on top of v4 patch?
> This will avoid releasing the lock and take care of the NEWLY_IDLE case.
> 
> Tim
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 43c5ec039633..26179f4b77f6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11772,14 +11772,13 @@ 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;
> @@ -11916,9 +11915,9 @@ 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);
> -
> +				if (env->idle == CPU_NEWLY_IDLE &&
> +				    (env->dst_running > 0 || env->dst_rq->ttwu_pending))
> +					goto out;

IIUC, we come here, it means busiest cpu was found, but due to
affinity restrictions none of the tasks can come to this cpu.

So a redo is done excluding that busiest cpu if there are cpus other
than the group_mask of this cpu. So doing a redo does make sense specially
for newidle.

So doing bailing out might be wrong.

>   				goto redo;
>   			}
>   			goto out_all_pinned;