Re: [PATCH] sched/fair: Optimize select_idle_core

[Valentin Schneider <valentin.schneider@arm.com>]

On 05/12/2019 17:23, Srikar Dronamraju wrote:
> Currently we loop through all threads of a core to evaluate if the core
> is idle or not. This is unnecessary. If a thread of a core is not
> idle, skip evaluating other threads of a core.
> 
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69a81a5709ff..b9d628128cfc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5872,10 +5872,12 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>  		bool idle = true;
>  
>  		for_each_cpu(cpu, cpu_smt_mask(core)) {
> -			__cpumask_clear_cpu(cpu, cpus);
> -			if (!available_idle_cpu(cpu))
> +			if (!available_idle_cpu(cpu)) {
>  				idle = false;
> +				break;
> +			}
>  		}
> +		cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>  

That looks sensible enough to me. I do have one random thought, however.


Say you have a 4-core SMT2 system with the usual numbering scheme:

{0, 4}  {1, 5}  {2, 6}  {3, 7}
CORE0   CORE1   CORE2   CORE3


Say 'target' is the prev_cpu, in that case let's pick 5. Because we do a
for_each_cpu_wrap(), our iteration for 'core' would start with 

  5, 6, 7, ...

So say CORE2 is entirely idle and CORE1 isn't, we would go through the
inner loop on CORE1 (with 'core' == 5), then go through CORE2 (with
'core' == 6) and return 'core'. I find it a bit unusual that we wouldn't
return the first CPU in the SMT mask, usually we try to fill sched_groups
in cpumask order.


If we could have 'cpus' start with only primary CPUs, that would simplify
things methinks:

  for_each_cpu_wrap(core, cpus, target) {
	  bool idle = true;

	  for_each_cpu(cpu, cpu_smt_mask(core)) {
		  if (!available_idle_cpu(cpu)) {
			  idle = false;
			  break;
		  }

	  __cpumask_clear_cpu(core, cpus);

	  if (idle)
		  return core;


Food for thought; your change itself looks fine as it is.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>


>  		if (idle)
>  			return core;
>