Re: [RFC PATCH 4/7] sched/fair: Calculate the scan depth for idle balance based on system utilization

[Shrikanth Hegde <sshegde@linux.vnet.ibm.com>]

On 7/27/23 8:05 PM, Chen Yu wrote:
> When the CPU is about to enter idle, it invokes newidle_balance()
> to pull some tasks from other runqueues. Although there is per
> domain max_newidle_lb_cost to throttle the newidle_balance(), it
> would be good to further limit the scan based on overall system
> utilization. The reason is that there is no limitation for
> newidle_balance() to launch this balance simultaneously on
> multiple CPUs. Since each newidle_balance() has to traverse all
> the groups to calculate the statistics one by one, this total
> time cost on newidle_balance() could be O(n^2). n is the number
> of groups. This issue is more severe if there are many groups
> within 1 domain, for example, a system with a large number of
> Cores in a LLC domain. This is not good for performance or
> power saving.
> 
> sqlite has spent quite some time on newidle balance() on Intel
> Sapphire Rapids, which has 2 x 56C/112T = 224 CPUs:
> 6.69%    0.09%  sqlite3     [kernel.kallsyms]   [k] newidle_balance
> 5.39%    4.71%  sqlite3     [kernel.kallsyms]   [k] update_sd_lb_stats
> 
> Based on this observation, limit the scan depth of newidle_balance()
> by considering the utilization of the sched domain. Let the number of
> scanned groups be a linear function of the utilization ratio:
> 
> nr_groups_to_scan = nr_groups * (1 - util_ratio)
> 
> Suggested-by: Tim Chen <tim.c.chen@intel.com>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
>  include/linux/sched/topology.h |  1 +
>  kernel/sched/fair.c            | 30 ++++++++++++++++++++++++++++++
>  kernel/sched/features.h        |  1 +
>  3 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index d6a64a2c92aa..af2261308529 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -84,6 +84,7 @@ struct sched_domain_shared {
>  	int		nr_idle_scan;
>  	unsigned long	total_load;
>  	unsigned long	total_capacity;
> +	int		nr_sg_scan;
>  };
>  
>  struct sched_domain {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index edcfee9965cd..6925813db59b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10153,6 +10153,35 @@ static void ilb_save_stats(struct lb_env *env,
>  		WRITE_ONCE(sd_share->total_capacity, sds->total_capacity);
>  }
>  
> +static void update_ilb_group_scan(struct lb_env *env,
> +				  unsigned long sum_util,
> +				  struct sched_domain_shared *sd_share)
> +{
> +	u64 tmp, nr_scan;
> +
> +	if (!sched_feat(ILB_UTIL))
> +		return;
> +
> +	if (!sd_share)
> +		return;
> +
> +	if (env->idle == CPU_NEWLY_IDLE)
> +		return;


Suggestion for small improvement:

First if condition here could be check for newidle. As it often very often we could save a few cycles of checking
sched feature.

> +	if (env->idle == CPU_NEWLY_IDLE)
> +		return;


> +
> +	/*
> +	 * Limit the newidle balance scan depth based on overall system
> +	 * utilization:
> +	 * nr_groups_scan = nr_groups * (1 - util_ratio)
> +	 * and util_ratio = sum_util / (sd_weight * SCHED_CAPACITY_SCALE)
> +	 */
> +	nr_scan = env->sd->nr_groups * sum_util;
> +	tmp = env->sd->span_weight * SCHED_CAPACITY_SCALE;
> +	do_div(nr_scan, tmp);
> +	nr_scan = env->sd->nr_groups - nr_scan;
> +	if ((int)nr_scan != sd_share->nr_sg_scan)
> +		WRITE_ONCE(sd_share->nr_sg_scan, (int)nr_scan);
> +}
> +
>  /**
>   * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
>   * @env: The load balancing environment.
> @@ -10231,6 +10260,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	}
>  
>  	update_idle_cpu_scan(env, sum_util);
> +	update_ilb_group_scan(env, sum_util, sd_share);
>  
>  	/* save a snapshot of stats during periodic load balance */
>  	ilb_save_stats(env, sd_share, sds);
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 3cb71c8cddc0..30f6d1a2f235 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -103,3 +103,4 @@ SCHED_FEAT(ALT_PERIOD, true)
>  SCHED_FEAT(BASE_SLICE, true)
>  
>  SCHED_FEAT(ILB_SNAPSHOT, true)
> +SCHED_FEAT(ILB_UTIL, true)