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

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

On 10/14/25 2:54 PM, Peter Zijlstra wrote:

> 
> Should we instead move the whole serialize thing inside
> sched_balance_rq() like so:
> 
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bc0b7ce8a65d..e9f719ba17e1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11722,6 +11722,22 @@ static void update_lb_imbalance_stat(struct lb_env *env, struct sched_domain *sd
>   	}
>   }
>   
> +
> +/*
> + * This flag serializes load-balancing passes over large domains
> + * (above the NODE topology level) - only one load-balancing instance
> + * may run at a time, to reduce overhead on very large systems with
> + * lots of CPUs and large NUMA distances.
> + *
> + * - Note that load-balancing passes triggered while another one
> + *   is executing are skipped and not re-tried.
> + *
> + * - Also note that this does not serialize rebalance_domains()
> + *   execution, as non-SD_SERIALIZE domains will still be
> + *   load-balanced in parallel.
> + */
> +static atomic_t sched_balance_running = ATOMIC_INIT(0);
> +
>   /*
>    * Check this_cpu to ensure it is balanced within domain. Attempt to move
>    * tasks if there is an imbalance.
> @@ -11747,6 +11763,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>   		.fbq_type	= all,
>   		.tasks		= LIST_HEAD_INIT(env.tasks),
>   	};
> +	int need_unlock = false;
>   
>   	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>   
> @@ -11758,6 +11775,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>   		goto out_balanced;
>   	}
>   
> +	if (idle != CPU_NEWLY_IDLE && (sd->flags & SD_SERIALIZE)) {
> +		if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
> +			goto out_balanced;
> +		need_unlock = true;
> +	}
> +
>   	group = sched_balance_find_src_group(&env);
>   	if (!group) {
>   		schedstat_inc(sd->lb_nobusyg[idle]);
> @@ -11998,6 +12021,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
>   	    sd->balance_interval < sd->max_interval)
>   		sd->balance_interval *= 2;
>   out:
> +	if (need_unlock)
> +		atomic_set_release(&sched_balance_running, 0);
> +
>   	return ld_moved;
>   }
>   
> @@ -12122,21 +12148,6 @@ static int active_load_balance_cpu_stop(void *data)
>   	return 0;
>   }
>   
> -/*
> - * This flag serializes load-balancing passes over large domains
> - * (above the NODE topology level) - only one load-balancing instance
> - * may run at a time, to reduce overhead on very large systems with
> - * lots of CPUs and large NUMA distances.
> - *
> - * - Note that load-balancing passes triggered while another one
> - *   is executing are skipped and not re-tried.
> - *
> - * - Also note that this does not serialize rebalance_domains()
> - *   execution, as non-SD_SERIALIZE domains will still be
> - *   load-balanced in parallel.
> - */
> -static atomic_t sched_balance_running = ATOMIC_INIT(0);
> -
>   /*
>    * Scale the max sched_balance_rq interval with the number of CPUs in the system.
>    * This trades load-balance latency on larger machines for less cross talk.
> @@ -12192,7 +12203,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>   	/* Earliest time when we have to do rebalance again */
>   	unsigned long next_balance = jiffies + 60*HZ;
>   	int update_next_balance = 0;
> -	int need_serialize, need_decay = 0;
> +	int need_decay = 0;
>   	u64 max_cost = 0;
>   
>   	rcu_read_lock();
> @@ -12216,13 +12227,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>   		}
>   
>   		interval = get_sd_balance_interval(sd, busy);
> -
> -		need_serialize = sd->flags & SD_SERIALIZE;
> -		if (need_serialize) {
> -			if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
> -				goto out;
> -		}
> -
>   		if (time_after_eq(jiffies, sd->last_balance + interval)) {
>   			if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
>   				/*
> @@ -12236,9 +12240,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>   			sd->last_balance = jiffies;
>   			interval = get_sd_balance_interval(sd, busy);
>   		}
> -		if (need_serialize)
> -			atomic_set_release(&sched_balance_running, 0);
> -out:
> +
>   		if (time_after(next_balance, sd->last_balance + interval)) {
>   			next_balance = sd->last_balance + interval;
>   			update_next_balance = 1;

One thing is missing, need to reset to 0 for redo logic to work.

@@ -11882,6 +11905,8 @@ 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);
                                 goto redo;