Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling

[Rohit Jain]

On 02/09/2018 04:53 AM, Peter Zijlstra wrote:
> On Mon, Jan 29, 2018 at 03:27:09PM -0800, Rohit Jain wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 26a71eb..ce5ccf8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu)
> >   	return cpu_rq(cpu)->cpu_capacity_orig;
> >   }
> >   
> > +static inline bool full_capacity(int cpu)
> > +{
> > +	return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
> > +}
> I don't like that name; >.75 != 1.
>
> Maybe invert things and do something like:
>
> static inline bool reduced_capacity(int cpu)
> {
> 	return capacity_of(cpu) < (3*capacity_orig_of(cpu))/4;
> }

OK, I will change the name and invert the logic.

> > @@ -6110,11 +6116,13 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
> >   	for_each_cpu(cpu, cpu_smt_mask(target)) {
> >   		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> >   			continue;
> > +		if (idle_cpu(cpu) && (capacity_of(cpu) > max_cap)) {
> > +			max_cap = capacity_of(cpu);
> > +			rcpu = cpu;
> > +		}
> 		if (idle_cpu(cpu)) {
> 			if (!reduced_capacity(cpu))
> 				return cpu;
>
> 			if (capacity_cpu(cpu) > max_cap) {
> 				max_cap = capacity_cpu(cpu);
> 				rcpu = cpu;
> 			}
> 		}
>
> Would be more consistent, I think.

OK

> >   	}
> >   
> > -	return -1;
> > +	return rcpu;
> >   }
>
>
> > @@ -6143,6 +6151,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >   	u64 time, cost;
> >   	s64 delta;
> >   	int cpu, nr = INT_MAX;
> > +	int best_cpu = -1;
> > +	unsigned int best_cap = 0;
> Randomly different names for the same thing as in select_idle_smt().
> Thinking up two different names for the same thing is more work; be more
> lazy.

OK, will be more consistent in v1

> >   	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> >   	if (!this_sd)
> > @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >   			return -1;
> >   		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> >   			continue;
> > +		if (idle_cpu(cpu)) {
> > +			if (full_capacity(cpu)) {
> > +				best_cpu = cpu;
> > +				break;
> > +			} else if (capacity_of(cpu) > best_cap) {
> > +				best_cap = capacity_of(cpu);
> > +				best_cpu = cpu;
> > +			}
> > +		}
> No need for the else. And you'll note you're once again inconsistent
> with your previous self.
>
> But here I worry about big.little a wee bit. I think we're allowed big
> and little cores on the same L3 these days, and you can't directly
> compare capacity between them.
>
> Morten / Dietmar, any comments?
>
> > @@ -6193,13 +6210,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >   	struct sched_domain *sd;
> >   	int i;
> >   
> > -	if (idle_cpu(target))
> > +	if (idle_cpu(target) && full_capacity(target))
> >   		return target;
> >   
> >   	/*
> >   	 * If the previous cpu is cache affine and idle, don't be stupid.
> >   	 */
> > -	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
> > +	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev) &&
> > +	    full_capacity(prev))
> >   		return prev;
> split before idle_cpu() for a better balance.


OK

Thanks,
Rohit