Re: [PATCH v5 0/2] sched/fair: Optimize some active balance logic

[Aiqun(Maria) Yu]

On 6/18/2026 6:56 PM, Peter Zijlstra wrote:
> 
> 
> And since I've been staring at this code far too long, I accidentally
> did the below cleanup on top.
> 
> 
> ---
> Subject: sched/fair: Reflow sched_balance_rq()
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Thu Jun 18 10:51:49 CEST 2026
> 
> Reflow to reduce indenting.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c  |  136 ++++++++++++++++++++++++---------------------------
>  kernel/sched/sched.h |   19 ++++++-
>  2 files changed, 82 insertions(+), 73 deletions(-)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13437,82 +13437,78 @@ static int sched_balance_rq(int this_cpu
>  		}
>  	}
>  
> -	if (!ld_moved) {
> -		schedstat_inc(sd->lb_failed[idle]);
> +	if (ld_moved) {
> +		sd->nr_balance_failed = 0;
> +		goto out_unbalanced;
> +	}
> +
> +	schedstat_inc(sd->lb_failed[idle]);
> +	/*
> +	 * Increment the failure counter only on periodic balance.
> +	 * We do not want newidle balance, which can be very
> +	 * frequent, pollute the failure counter causing
> +	 * excessive cache_hot migrations and active balances.
> +	 *
> +	 * Similarly for migration_misfit which is not related to
> +	 * load/util migration, don't pollute nr_balance_failed.
> +	 *
> +	 * The same for cache aware scheduling's allowance for
> +	 * load imbalance. If regular load balance does not
> +	 * migrate task due to LLC locality, it is a expected
> +	 * behavior and don't pollute nr_balance_failed.
> +	 * See can_migrate_task().
> +	 */
> +	if (idle != CPU_NEWLY_IDLE &&
> +	    env.migration_type != migrate_misfit &&
> +	    !(env.flags & LBF_LLC_PINNED))
> +		sd->nr_balance_failed++;
> +
> +	if (!need_active_balance(&env))
> +		goto out_unbalanced;
> +
> +	scoped_guard (raw_spin_rq_lock_irqsave, busiest) {
>  		/*
> -		 * Increment the failure counter only on periodic balance.
> -		 * We do not want newidle balance, which can be very
> -		 * frequent, pollute the failure counter causing
> -		 * excessive cache_hot migrations and active balances.
> -		 *
> -		 * Similarly for migration_misfit which is not related to
> -		 * load/util migration, don't pollute nr_balance_failed.
> -		 *
> -		 * The same for cache aware scheduling's allowance for
> -		 * load imbalance. If regular load balance does not
> -		 * migrate task due to LLC locality, it is a expected
> -		 * behavior and don't pollute nr_balance_failed.
> -		 * See can_migrate_task().
> +		 * Don't kick the active_load_balance_cpu_stop,
> +		 * if the curr task on busiest CPU can't be
> +		 * moved to this_cpu:
>  		 */
> -		if (idle != CPU_NEWLY_IDLE &&
> -		    env.migration_type != migrate_misfit &&
> -		    !(env.flags & LBF_LLC_PINNED))
> -			sd->nr_balance_failed++;
> -
> -		if (need_active_balance(&env)) {
> -			unsigned long flags;
> -
> -			raw_spin_rq_lock_irqsave(busiest, flags);
> -
> -			/*
> -			 * Don't kick the active_load_balance_cpu_stop,
> -			 * if the curr task on busiest CPU can't be
> -			 * moved to this_cpu:
> -			 */
> -			if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
> -				raw_spin_rq_unlock_irqrestore(busiest, flags);
> -				goto out_one_pinned;
> -			}
> -
> -			/* Record that we found at least one task that could run on this_cpu */
> -			env.flags &= ~LBF_ALL_PINNED;
> -
> -			/*
> -			 * ->active_balance synchronizes accesses to
> -			 * ->active_balance_work.  Once set, it's cleared
> -			 * only after active load balance is finished.
> -			 */
> -			if (busiest->active_balance)
> -				goto no_active_balance;
> -
> -			/*
> -			 * @busiest dropped its rq_lock in the middle of
> -			 * scheduling out its ->curr task (->on_rq := 0), no
> -			 * need to forcefully punt it away with active balance.
> -			 */
> -			if (!busiest->curr->on_rq)
> -				goto no_active_balance;
> -
> -			busiest->active_balance = 1;
> -			busiest->push_cpu = this_cpu;
> -			active_balance = 1;
> -no_active_balance:
> -			preempt_disable();
> -			raw_spin_rq_unlock_irqrestore(busiest, flags);
> -			if (active_balance) {
> -				stop_one_cpu_nowait(cpu_of(busiest),
> -					active_load_balance_cpu_stop, busiest,
> -					&busiest->active_balance_work);
> -			}
> -			preempt_enable();
> -		}
> -	} else {
> -		sd->nr_balance_failed = 0;
> +		if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr))
> +			goto out_one_pinned;
> +
> +		/* Record that we found at least one task that could run on this_cpu */
> +		env.flags &= ~LBF_ALL_PINNED;
> +
> +		/*
> +		 * ->active_balance synchronizes accesses to
> +		 * ->active_balance_work.  Once set, it's cleared
> +		 * only after active load balance is finished.
> +		 */
> +		if (busiest->active_balance)
> +			goto out_unbalanced;
> +
> +		/*
> +		 * @busiest dropped its rq_lock in the middle of
> +		 * scheduling out its ->curr task (->on_rq := 0), no
> +		 * need to forcefully punt it away with active balance.
> +		 */
> +		if (!busiest->curr->on_rq)

This looks better than the previous patch.

While at it, should we check busiest->cfs.h_nr_queued instead of
!busiest->curr->on_rq?

It is possible that busiest->curr is an RT/DL task and is still on_rq
while busiest->cfs.h_nr_queued == 0 and list_empty(&busiest->cfs_tasks)
may both be true, meaning there is no CFS task available for active
balancing.

This series has been moving quickly, and I only got back from the Dragon
Boat Festival recently. I should have raised this earlier, but I am
commenting here since this is now the clean top patch.

> +			goto out_unbalanced;
> +
> +		busiest->active_balance = 1;
> +		busiest->push_cpu = this_cpu;
> +		active_balance = 1;
> +		preempt_disable();
>  	}
> +	if (active_balance) {
> +		stop_one_cpu_nowait(cpu_of(busiest),
> +				    active_load_balance_cpu_stop, busiest,
> +				    &busiest->active_balance_work);
> +	}
> +	preempt_enable();
>  
> +out_unbalanced:
>  	/* We were unbalanced, so reset the balancing interval */
>  	sd->balance_interval = sd->min_interval;
> -
>  	goto out;
>  
>  out_balanced:
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2018,7 +2018,8 @@ DEFINE_LOCK_GUARD_1(rq_lock, struct rq,
>  		    rq_unlock(_T->lock, &_T->rf),
>  		    struct rq_flags rf)
>  
> -DECLARE_LOCK_GUARD_1_ATTRS(rq_lock, __acquires(__rq_lockp(_T)), __releases(__rq_lockp(*(struct rq **)_T)));
> +DECLARE_LOCK_GUARD_1_ATTRS(rq_lock, __acquires(__rq_lockp(_T)),
> +			   __releases(__rq_lockp(*(struct rq **)_T)));
>  #define class_rq_lock_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock, _T)
>  
>  DEFINE_LOCK_GUARD_1(rq_lock_irq, struct rq,
> @@ -2026,7 +2027,8 @@ DEFINE_LOCK_GUARD_1(rq_lock_irq, struct
>  		    rq_unlock_irq(_T->lock, &_T->rf),
>  		    struct rq_flags rf)
>  
> -DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irq, __acquires(__rq_lockp(_T)), __releases(__rq_lockp(*(struct rq **)_T)));
> +DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irq, __acquires(__rq_lockp(_T)),
> +			   __releases(__rq_lockp(*(struct rq **)_T)));
>  #define class_rq_lock_irq_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock_irq, _T)
>  
>  DEFINE_LOCK_GUARD_1(rq_lock_irqsave, struct rq,
> @@ -2034,9 +2036,20 @@ DEFINE_LOCK_GUARD_1(rq_lock_irqsave, str
>  		    rq_unlock_irqrestore(_T->lock, &_T->rf),
>  		    struct rq_flags rf)
>  
> -DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irqsave, __acquires(__rq_lockp(_T)), __releases(__rq_lockp(*(struct rq **)_T)));
> +DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irqsave, __acquires(__rq_lockp(_T)),
> +			   __releases(__rq_lockp(*(struct rq **)_T)));
>  #define class_rq_lock_irqsave_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock_irqsave, _T)
>  
> +DEFINE_LOCK_GUARD_1(raw_spin_rq_lock_irqsave, struct rq,
> +		    raw_spin_rq_lock_irqsave(_T->lock, _T->flags),
> +		    raw_spin_rq_unlock_irqrestore(_T->lock, _T->flags),
> +		    unsigned long flags)
> +
> +DECLARE_LOCK_GUARD_1_ATTRS(raw_spin_rq_lock_irqsave, __acquires(__rq_lockp(_T)),
> +			   __releases(__rq_lockp(*(struct rq **)_T)));
> +#define class_raw_spin_rq_lock_irqsave_constructor(_T) \
> +	WITH_LOCK_GUARD_1_ATTRS(raw_spin_rq_lock_irqsave, _T)
> +
>  #define this_rq_lock_irq(...) __acquire_ret(_this_rq_lock_irq(__VA_ARGS__), __rq_lockp(__ret))
>  static inline struct rq *_this_rq_lock_irq(struct rq_flags *rf) __acquires_ret
>  {


-- 
Thx and BRs,
Aiqun(Maria) Yu