linux-kernel - Re: [RESEND RFC PATCH v2 28/29] [EXPERIMENTAL] sched/fair: Add a local counter to rate limit task push

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Message-ID: <861ae06a-5c9a-47ce-a290-709785b28833@arm.com>
Date: Mon, 8 Dec 2025 12:33:53 +0000
From: Christian Loehle <christian.loehle@....com>
To: K Prateek Nayak <kprateek.nayak@....com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Dietmar Eggemann
 <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Valentin Schneider <vschneid@...hat.com>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>,
 Swapnil Sapkal <swapnil.sapkal@....com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>, Chen Yu <yu.c.chen@...el.com>
Subject: Re: [RESEND RFC PATCH v2 28/29] [EXPERIMENTAL] sched/fair: Add a
 local counter to rate limit task push

On 12/8/25 09:27, K Prateek Nayak wrote:
> Pushing tasks can fail for multitude of reasons - task affinity, the
> unavailability of an idle CPUs by the time balance callback is executed,
> etc.
> 
> Maintain a CPU local counter in sched_domain to rate limit push attempts
> if the failures build up. This counter is reset at the time of periodic
> balance to the value in "nr_idle_scan".
> 
> Since "nr_idle_scan" is only computed for SIS_UTIL, rate limiting has
> been guarded behind the same sched_feat().
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
> ---
>  include/linux/sched/topology.h |  4 ++++
>  kernel/sched/fair.c            | 23 +++++++++++++++++++++--
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 074ee2980cdf..ebe26ce82c1a 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -122,6 +122,10 @@ struct sched_domain {
>  	unsigned int alb_failed;
>  	unsigned int alb_pushed;
>  
> +	/* Push load balancing */
> +	unsigned long last_nr_push_update;
> +	int nr_push_attempt;
> +
>  	/* SD_BALANCE_EXEC stats */
>  	unsigned int sbe_count;
>  	unsigned int sbe_balanced;
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 34aeb8e58e0b..46d33ab63336 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12356,6 +12356,16 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>  		rq->max_idle_balance_cost =
>  			max((u64)sysctl_sched_migration_cost, max_cost);
>  	}
> +	if (sched_feat(SIS_UTIL)) {
> +		sd = rcu_dereference(per_cpu(sd_llc, cpu));
> +
> +		if (sd && sd->shared &&
> +		    time_after_eq(jiffies, sd->last_nr_push_update + sd->min_interval)) {
> +			sd->nr_push_attempt = READ_ONCE(sd->shared->nr_idle_scan);
> +			sd->last_nr_push_update = jiffies;
> +		}
> +	}
> +
>  	rcu_read_unlock();
>  
>  	/*
> @@ -13110,8 +13120,6 @@ static inline bool should_push_tasks(struct rq *rq)
>  	struct sched_domain *sd;
>  	int cpu = cpu_of(rq);
>  
> -	/* TODO: Add a CPU local failure counter. */
> -
>  	/* CPU doesn't have any fair task to push. */
>  	if (!has_pushable_tasks(rq))
>  		return false;
> @@ -13126,6 +13134,10 @@ static inline bool should_push_tasks(struct rq *rq)
>  	if (!sd)
>  		return false;
>  
> +	/* We've failed to push task too many times. */
> +	if (sched_feat(SIS_UTIL) && sd->nr_push_attempt <= 0)
> +		return false;
> +
>  	/*
>  	 * We may not be able to find a push target.
>  	 * Skip for this tick and depend on the periodic
> @@ -13176,6 +13188,13 @@ static bool push_fair_task(struct rq *rq)
>  		return true;
>  	}
>  
> +	/*
> +	 * If the push failed after a full search, decrement the
> +	 * attempt counter to dicourage further attempts. Periodic
> +	 * balancer will reset the "nr_push_attempt" after a while.
> +	 */
> +	sd->nr_push_attempt--;
> +
>  	return false;
>  }
>  

Just to confirm, but this patch is included when the cover letter mentions "push" for the
benchmarks?
Did this help the regressions then?