Re: [PATCH] sched/fair: Distribute burst-woken tasks when rq depth reaches 3x average

[Kayra Cizmeci]

Hi!

On Wed, Nov 04, 2026 at 05:18:42PM +0800, Li kunyu wrote:

FWIW, This doesn't apply to the mainline:

Patch failed at 0001 sched/fair: Distribute burst-woken tasks when rq depth reaches 3x average
error: patch failed: kernel/sched/fair.c:9972
error: kernel/sched/fair.c: patch does not apply

> When a task burst-wakes a large number of tasks in a short period, tasks
> can pile up on a single CPU regardless of whether the system has idle
> CPUs or NO_HZ enabled.
> 
> Track the average runqueue depth (scaled by SCHED_CAPACITY_SCALE to
> preserve fractional precision) in root_domain during the periodic
> load-balance pass.
> 
> In select_task_rq_fair(), when the target runqueue depth reaches 3x the
> system-wide average, fall back to cpumask_any_and_distribute(p->cpus_ptr,
> cpu_active_mask) to rotate waking tasks across allowed CPUs and reset the
> periodic load balance intervals so regular load balancing can promptly
> smooth out the distribution.
> 
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/20261021125419.GK2823843@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Li kunyu <likunyu@xxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c     | 70 ++++++++++++++++++++++++++++++++++++++---
>  kernel/sched/sched.h    |  3 ++
>  kernel/sched/topology.c |  2 ++
>  3 files changed, 71 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6e005f1e8dc9..e4d278ac46c4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9903,6 +9903,38 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  	return target;
>  }
>  
> +/*
> + * Check if a CPU's rq depth reaches 3x the system-wide average tracked
> + * in root_domain (scaled by SCHED_CAPACITY_SCALE, floored at 1 task).
> + */
> +static inline bool rq_burst_overloaded(int cpu)
> +{
> +	struct rq *rq = cpu_rq(cpu);
> +	unsigned long avg;
> +
> +	if (unlikely(!rq->rd))
> +		return false;
> +
> +	avg = max_t(unsigned long, READ_ONCE(rq->rd->avg_nr_running),
> +		    SCHED_CAPACITY_SCALE);
> +
> +	return (READ_ONCE(rq->nr_running) << SCHED_CAPACITY_SHIFT) >= avg * 3;
> +}
> +
> +static inline void reset_periodic_lb_interval(int cpu)
> +{
> +	struct sched_domain *sd;
> +
> +	/*
> +	 * Only the most basic values are updated here to avoid excessive
> +	 * cost.
> +	 */
> +	for_each_domain(cpu, sd) {
> +		if (READ_ONCE(sd->balance_interval) > sd->min_interval)
> +			WRITE_ONCE(sd->balance_interval, sd->min_interval);
> +	}
> +}
> +
>  /*
>   * select_task_rq_fair: Select target runqueue for the waking task in domains
>   * that have the relevant SD flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -9972,11 +10004,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  
>  	/* Slow path */
>  	if (unlikely(sd))
> -		return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> -
> +		new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
>  	/* Fast path */
> -	if (wake_flags & WF_TTWU)
> -		return select_idle_sibling(p, prev_cpu, new_cpu);
> +	else if (wake_flags & WF_TTWU)
> +		new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> +
> +	if (unlikely(rq_burst_overloaded(new_cpu))) {
> +		int dist_cpu = cpumask_any_and_distribute(p->cpus_ptr, cpu_active_mask);
> +
> +		if (likely(dist_cpu < nr_cpumask_bits)) {
> +			reset_periodic_lb_interval(new_cpu);

Why are we calling this for new_cpu?

> +			return dist_cpu;
> +		}
> +	}
>  
>  	return new_cpu;
>  }
> @@ -11685,6 +11725,7 @@ struct sg_lb_stats {
>  struct sd_lb_stats {
>  	struct sched_group *busiest;		/* Busiest group in this sd */
>  	struct sched_group *local;		/* Local group in this sd */
> +	unsigned long total_nr_running;		/* Sum of nr_running in this sd */
>  	unsigned long total_load;		/* Total load of all groups in sd */
>  	unsigned long total_capacity;		/* Total capacity of all groups in sd */
>  	unsigned long avg_load;			/* Average load across all groups in sd */
> @@ -12964,6 +13005,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  		/* Now, start updating sd_lb_stats */
>  		sds->total_load += sgs->group_load;
>  		sds->total_capacity += sgs->group_capacity;
> +		sds->total_nr_running += sgs->sum_nr_running;
>  
>  		sum_util += sgs->group_util;
>  		sg = sg->next;
> @@ -12982,11 +13024,31 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>  
>  	if (!env->sd->parent) {
> +		unsigned int weight = env->sd->span_weight;

Uh-oh. There's something wrong, no?

span_weight equals all of the CPU's in the domain.
So in sched_balance_rq() we call sched_balance_find_src_group() and that
calls update_sd_lb_stats().

But it just retries balancing when all of the tasks
in the rq is pinned. In that case the CPU is cleared from the env->cpus
bitmap. But sum_nr_running is calculated for all env->cpus while
span_weight is for all of the CPU's in that domain.

BTW, in theory this could happen multiple times in a row and
it could make things even worse.


Or am I missing something?


> +
>  		/* update overload indicator if we are at root domain */
>  		set_rd_overloaded(env->dst_rq->rd, sg_overloaded);
>  
>  		/* Update over-utilization (tipping point, U >= 0) indicator */
>  		set_rd_overutilized(env->dst_rq->rd, sg_overutilized);
> +
> +		/*
> +		 * We update the average task count at the top-level (root)
> +		 * domain, as this incurs the least overhead and comes
> +		 * closest to the ideal scenario. The trade-off is that we
> +		 * must accept some temporary data inaccuracy.
> +		 */
> +		if (weight) {
> +			unsigned long cur_avg;
> +			unsigned long old_avg;
> +
> +			cur_avg = (sds->total_nr_running << SCHED_CAPACITY_SHIFT) / weight;
> +			old_avg = READ_ONCE(env->dst_rq->rd->avg_nr_running);
> +			if (old_avg)
> +				cur_avg = (old_avg * 7 + cur_avg) >> 3;
> +
> +			WRITE_ONCE(env->dst_rq->rd->avg_nr_running, cur_avg);
> +		}
>  	} else if (sg_overutilized) {
>  		set_rd_overutilized(env->dst_rq->rd, sg_overutilized);
>  	}
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e656c7059bf8..4147e043407c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1077,6 +1077,9 @@ struct root_domain {
>  	cpumask_var_t		rto_mask;
>  	struct cpupri		cpupri;
>  
> +	/* Scaled (SCHED_CAPACITY_SCALE) average per-CPU rq depth */
> +	unsigned long		avg_nr_running;
> +
>  	/*
>  	 * NULL-terminated list of performance domains intersecting with the
>  	 * CPUs of the rd. Protected by RCU.
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 3dab0253976f..41c3b9e30d65 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -558,6 +558,8 @@ static int init_rootdomain(struct root_domain *rd)
>  
>  	if (cpupri_init(&rd->cpupri) != 0)
>  		goto free_cpudl;
> +
> +	rd->avg_nr_running = SCHED_CAPACITY_SCALE;
>  	return 0;
>  
>  free_cpudl:
> -- 
> 2.47.3
> 

BTW, if we are looking to nr_running for this how about atleast looking
at our dist_cpu and not returning it if its nr_running is bigger
than the other CPU. Right now we are looking only at the average.

Would this work for you? Peter/Li?


Thanks,

Kayra