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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 14, 2025 at 07:20:35PM +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@infradead.org> [2025-10-14 11:24:36]:
> 
> > On Mon, Oct 13, 2025 at 02:54:19PM -0700, Tim Chen wrote:
> > 
> > 
> > Right, Yu Chen said something like that as well, should_we_balance() is
> > too late.
> > 
> > Should we instead move the whole serialize thing inside
> > sched_balance_rq() like so:
> > 
> > @@ -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;
> 
> I think this is better since previously the one CPU which was not suppose to
> do the balancing may increment the atomic variable. If the CPU, that was
> suppose to do the balance now tries it may fail since the variable was not
> yet decremented.

Right, it would do that acquire and then still have at least 2 ways to
not actually balance, which is a waste.