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 3:12 PM, Peter Zijlstra wrote:
> On Tue, Oct 14, 2025 at 03:03:41PM +0530, Shrikanth Hegde wrote:
> 
>>> @@ -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;
>>
>> Maybe goto out instead of out_balanced ?
> 
> That would be inconsistent with the !should_we_balance() goto
> out_balanced right above this, no?

Yes. But whats the reason for saying out_balanced for !should_we_balance?

Load balance wasn't even attempted there right? Could this be updating it wrongly?
At-least comments around out_all_pinned doesn't make sense if we came here via !swb

         schedstat_inc(sd->lb_balanced[idle]);
         sd->nr_balance_failed = 0;


> 
>>> +		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;
>>