Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle_cpu()

[Chen Yu]

Hi Prateek,

On 2023-09-14 at 11:00:02 +0530, K Prateek Nayak wrote:
> Hello Chenyu,
> 
> One question ...
> 
> On 9/11/2023 8:20 AM, Chen Yu wrote:
> > [..snip..]
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e20f50726ab8..fe3b760c9654 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > [..more snip..]
> > @@ -7052,10 +7072,14 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> >  	int cpu;
> >  
> >  	for_each_cpu(cpu, cpu_smt_mask(core)) {
> > -		if (!available_idle_cpu(cpu)) {
> > +		bool cache_hot = sched_feat(SIS_CACHE) ?
> > +			sched_clock_cpu(cpu) < cpu_rq(cpu)->cache_hot_timeout : false;
> > +
> > +		if (!available_idle_cpu(cpu) || cache_hot) {
> >  			idle = false;
> >  			if (*idle_cpu == -1) {
> > -				if (sched_idle_cpu(cpu) && cpumask_test_cpu(cpu, p->cpus_ptr)) {
> > +				if (sched_idle_cpu(cpu) && cpumask_test_cpu(cpu, p->cpus_ptr) &&
> > +				    !cache_hot) {
> 
> Here, the CPU is running a SCHED_IDLE task ...
> 
> >  					*idle_cpu = cpu;
> >  					break;
> >  				}
> 
> ... but just below this, there are following lines to cache the idle_cpu:
> 
> 		}
> 		if (*idle_cpu == -1 && cpumask_test_cpu(cpu, p->cpus_ptr))
> 			*idle_cpu = cpu;
> 
> Would it make sense to also add the same "cache_hot" check here when we
> come across an idle CPU during the search for an idle core? Something
> like:
> 
> -		if (*idle_cpu == -1 && cpumask_test_cpu(cpu, p->cpus_ptr))

When we reached above code, the following condition should be true:
 (available_idle_cpu(cpu) && !cache_hot)
because the previous 'if' statement is false. So I guess we already
has !cache_hot ?

> +		if (*idle_cpu == -1 && !cache_hot && cpumask_test_cpu(cpu, p->cpus_ptr))
> 			*idle_cpu = cpu;
> 
> Implications with the above change:
> 
> If the entire core is idle, "select_idle_core()" will return the core
> and the search will bail out in "select_idle_cpu()". Otherwise, the
> cache-hot idle CPUs encountered during the search for idle core will be
> ignored now and if "idle_cpu" is not -1, it contains an idle CPU that is
> not cache-hot.
> 
> Thoughts?
> 

Yes, agree, we want to skip the cache-hot idle CPU if that entire core is not idle
in your case.

thanks,
Chenyu