linux-kernel - Re: [PATCH 1/5] sched/fair: Filter false overloaded

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Message-ID: <a499b2f6-eac3-4c15-bca6-001a724225ff@arm.com>
Date: Mon, 2 Sep 2024 10:01:09 +0100
From: Hongyan Xia <hongyan.xia2@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, lukasz.luba@....com, rafael.j.wysocki@...el.com,
 linux-kernel@...r.kernel.org
Cc: qyousef@...alina.io
Subject: Re: [PATCH 1/5] sched/fair: Filter false overloaded_group case for
 EAS

On 30/08/2024 14:03, Vincent Guittot wrote:
> With EAS, a group should be set overloaded if at least 1 CPU in the group
> is overutilized bit it can happen that a CPU is fully utilized by tasks
> because of clamping the compute capacity of the CPU. In such case, the CPU
> is not overutilized and as a result should not be set overloaded as well.
> 
> group_overloaded being a higher priority than group_misfit, such group can
> be selected as the busiest group instead of a group with a mistfit task
> and prevents load_balance to select the CPU with the misfit task to pull
> the latter on a fitting CPU.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   kernel/sched/fair.c | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fea057b311f6..e67d6029b269 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9806,6 +9806,7 @@ struct sg_lb_stats {
>   	enum group_type group_type;
>   	unsigned int group_asym_packing;	/* Tasks should be moved to preferred CPU */
>   	unsigned int group_smt_balance;		/* Task on busy SMT be moved */
> +	unsigned long group_overutilized;	/* No CPU is overutilized in the group */

Does this have to be unsigned long? I think a shorter width like bool 
(or int to be consistent with other fields) expresses the intention.

Also the comment to me is a bit confusing. All other fields are positive 
but this one's comment is in a negative tone.

>   	unsigned long group_misfit_task_load;	/* A CPU has a task too big for its capacity */
>   #ifdef CONFIG_NUMA_BALANCING
>   	unsigned int nr_numa_running;
> @@ -10039,6 +10040,13 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
>   static inline bool
>   group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
>   {
> +	/*
> +	 * With EAS and uclamp, 1 CPU in the group must be overutilized to
> +	 * consider the group overloaded.
> +	 */
> +	if (sched_energy_enabled() && !sgs->group_overutilized)
> +		return false;
> +
>   	if (sgs->sum_nr_running <= sgs->group_weight)
>   		return false;
>   
> @@ -10252,8 +10260,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>   		if (nr_running > 1)
>   			*sg_overloaded = 1;
>   
> -		if (cpu_overutilized(i))
> +		if (cpu_overutilized(i)) {
>   			*sg_overutilized = 1;
> +			sgs->group_overutilized = 1;
> +		}
>   
>   #ifdef CONFIG_NUMA_BALANCING
>   		sgs->nr_numa_running += rq->nr_numa_running;