Re: [PATCH 4/4] sched/fair: Proportional newidle balance

[Adam Li]

On 11/8/2025 12:06 AM, Peter Zijlstra wrote:
> Add a randomized algorithm that runs newidle balancing proportional to
> its success rate.
> 
> This improves schbench significantly:
> 
>  6.18-rc4:			2.22 Mrps/s
>  6.18-rc4+revert:		2.04 Mrps/s
>  6.18-rc4+revert+random:	2.18 Mrps/S
> 
> Conversely, per Adam Li this affects SpecJBB slightly, reducing it by 1%:
> 
>  6.17:			-6%
>  6.17+revert:		 0%
>  6.17+revert+random:	-1%
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Tested-by: Adam Li <adamli@xxxxxxxxxxxxxxxxxxxxxx>

Please see the Specjbb test result on AmpereOne server bellow:
6.18-rc5:			0% (baseline)
6.18-rc5+patchset:		+5%
6.18-rc4+patchset+NO_NI_RANDOM:	+6%
6.18-rc5+revert-155213a2aed4:	+6%

Could you please explain a little the math behind success rate
(sd->newidle_ratio) calculation?

[...]
> @@ -12843,6 +12858,22 @@ static int sched_balance_newidle(struct
>  			break;
>  
>  		if (sd->flags & SD_BALANCE_NEWIDLE) {
> +			unsigned int weight = 1;
> +
> +			if (sched_feat(NI_RANDOM)) {
> +				/*
> +				 * Throw a 1k sided dice; and only run
> +				 * newidle_balance according to the success
> +				 * rate.
> +				 */
> +				u32 d1k = sched_rng() % 1024;
> +				weight = 1 + sd->newidle_ratio;
> +				if (d1k > weight) {
> +					update_newidle_stats(sd, 0);
> +					continue;
> +				}
> +				weight = (1024 + weight/2) / weight;
> +			}
>  
e.g: Why 'weight = (1024 + weight/2) / weight'


Thanks,
-adam