linux-kernel - Re: [RESEND PATCH] sched/fair: Skip sched_balance

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Message-ID: <20251014094259.GR3245006@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 11:42:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	Chen Yu <yu.c.chen@...el.com>, Doug Nelson <doug.nelson@...el.com>,
	Mohini Narkhede <mohini.narkhede@...el.com>,
	linux-kernel@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg
 when balance is not due

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?

> > +		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;
>