Re: [RFC PATCH 5/7] sched/fair: Adjust the busiest group scanning depth in idle load balance

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

On 7/27/23 8:05 PM, Chen Yu wrote:
> Scanning the whole sched domain to find the busiest group is time costly
> during newidle_balance(). And if a CPU becomes idle, it would be good
> if this idle CPU pulls some tasks from other CPUs as quickly as possible.
> 
> Limit the scan depth of newidle_balance() to only scan for a limited number
> of sched groups to find a relatively busy group, and pull from it.
> In summary, the more spare time there is in the domain, the more time
> each newidle balance can spend on scanning for a busy group. Although
> the newidle balance has per domain max_newidle_lb_cost to decide
> whether to launch the balance or not, the ILB_UTIL provides a smaller
> granularity to decide how many groups each newidle balance can scan.
> 
> The scanning depth is calculated by the previous periodic load balance
> based on its overall utilization.
> 
> Tested on top of v6.5-rc2, Sapphire Rapids with 2 x 56C/112T = 224 CPUs.
> With cpufreq governor set to performance, and C6 disabled.
> 
> Firstly, tested on a extreme synthetic test[1], which launches 224
> process. Each process is a loop of nanosleep(1 us), which is supposed
> to trigger newidle balance as much as possible:
> 
> i=1;while [ $i -le "224" ]; do ./nano_sleep 1000 & i=$(($i+1)); done;
> 
> NO_ILB_UTIL + ILB_SNAPSHOT:
> 9.38%     0.45%  [kernel.kallsyms]   [k] newidle_balance
> 6.84%     5.32%  [kernel.kallsyms]   [k] update_sd_lb_stats.constprop.0
> 
> ILB_UTIL + ILB_SNAPSHOT:
> 3.35%     0.38%  [kernel.kallsyms]   [k] newidle_balance
> 2.30%     1.81%  [kernel.kallsyms]   [k] update_sd_lb_stats.constprop.0
> [...]

> Link: https://raw.githubusercontent.com/chen-yu-surf/tools/master/stress_nanosleep.c #1
> Suggested-by: Tim Chen <tim.c.chen@intel.com>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
>  kernel/sched/fair.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6925813db59b..4e360ed16e14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10195,7 +10195,13 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	struct sg_lb_stats *local = &sds->local_stat;
>  	struct sg_lb_stats tmp_sgs;
>  	unsigned long sum_util = 0;
> -	int sg_status = 0;
> +	int sg_status = 0, nr_sg_scan;
> +	/* only newidle CPU can load the snapshot */
> +	bool ilb_can_load = env->idle == CPU_NEWLY_IDLE &&
> +			    sd_share && READ_ONCE(sd_share->total_capacity);
> +
> +	if (sched_feat(ILB_UTIL) && ilb_can_load)

Suggestion for small improvement:

it could be ? This could help save a few cycles of checking if the feature is enabled when its not newidle. 

	if ( ilb_can_load && sched_feat(ILB_UTIL)) 

Same comments below in this patch as well in PATCH 6/7.

> +		nr_sg_scan = sd_share->nr_sg_scan;
>  
>  	do {
>  		struct sg_lb_stats *sgs = &tmp_sgs;
> @@ -10222,6 +10228,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  			sds->busiest_stat = *sgs;
>  		}
>  
> +		if (sched_feat(ILB_UTIL) && ilb_can_load && --nr_sg_scan <= 0)
> +			goto load_snapshot;
> +

Same comment as above.

>  next_group:
>  		/* Now, start updating sd_lb_stats */
>  		sds->total_load += sgs->group_load;
> @@ -10231,6 +10240,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  		sg = sg->next;
>  	} while (sg != env->sd->groups);
>  
> +	ilb_can_load = false;
> +
> +load_snapshot:
> +	if (ilb_can_load) {
> +		/* borrow the statistic of previous periodic load balance */
> +		sds->total_load = READ_ONCE(sd_share->total_load);
> +		sds->total_capacity = READ_ONCE(sd_share->total_capacity);
> +	}
> +
>  	/*
>  	 * Indicate that the child domain of the busiest group prefers tasks
>  	 * go to a child's sibling domains first. NB the flags of a sched group