Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan

[Pavan Kondeti]

On Tue, Jan 28, 2020 at 11:30:26AM +0000, Valentin Schneider wrote:
> Hi Pavan,
> 
> On 28/01/2020 06:22, Pavan Kondeti wrote:
> > Hi Valentin,
> > 
> > On Sun, Jan 26, 2020 at 08:09:32PM +0000, Valentin Schneider wrote:
> >>  
> >> +static inline int check_cpu_capacity(struct rq *rq, struct sched_domain *sd);
> >> +
> >> +/*
> >> + * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which
> >> + * the task fits. If no CPU is big enough, but there are idle ones, try to
> >> + * maximize capacity.
> >> + */
> >> +static int select_idle_capacity(struct task_struct *p, int target)
> >> +{
> >> +	unsigned long best_cap = 0;
> >> +	struct sched_domain *sd;
> >> +	struct cpumask *cpus;
> >> +	int best_cpu = -1;
> >> +	struct rq *rq;
> >> +	int cpu;
> >> +
> >> +	if (!static_branch_unlikely(&sched_asym_cpucapacity))
> >> +		return -1;
> >> +
> >> +	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target));
> >> +	if (!sd)
> >> +		return -1;
> >> +
> >> +	sync_entity_load_avg(&p->se);
> >> +
> >> +	cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> >> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >> +
> >> +	for_each_cpu_wrap(cpu, cpus, target) {
> >> +		rq = cpu_rq(cpu);
> >> +
> >> +		if (!available_idle_cpu(cpu))
> >> +			continue;
> >> +		if (task_fits_capacity(p, rq->cpu_capacity))
> >> +			return cpu;
> > 
> > I have couple of questions.
> > 
> > (1) Any particular reason for not checking sched_idle_cpu() as a backup
> > for the case where all eligible CPUs are busy? select_idle_cpu() does
> > that.
> > 
> 
> No particular reason other than we didn't consider it, I think. I don't see
> any harm in folding it in, I'll do that for v4. I am curious however; are
> you folks making use of SCHED_IDLE? AFAIA Android isn't making use of it
> yet, though Viresh paved the way for that to happen.
> 

We are not using SCHED_IDLE in product setups. I am told Android may use it
for background tasks in future. I am not completely sure though. I asked it
because select_idle_cpu() is using it.

> > (2) Assuming all CPUs are busy, we return -1 from here and end up
> > calling select_idle_cpu(). The traversal in select_idle_cpu() may be
> > waste in cases where sd_llc == sd_asym_cpucapacity . For example SDM845.
> > Should we worry about this?
> > 
> 
> Before v3, since we didn't have the fallback CPU selection within
> select_idle_capacity(), we would need the fall-through to select_idle_cpu()
> (we could've skipped an idle CPU just because its capacity wasn't high
> enough).
> 
> That's not the case anymore, so indeed we may be able to bail out of
> select_idle_sibling() right after select_idle_capacity() (or after the
> prev / recent_used_cpu checks). Our only requirement here is that sd_llc
> remains a subset of sd_asym_cpucapacity.
> 
> So far for Arm topologies we can have:
> - sd_llc < sd_asym_cpucapacity (e.g. legacy big.LITTLE like Juno)
> - sd_llc == sd_asym_cpucapacity (e.g. DynamIQ like SDM845)
> 
> I'm slightly worried about sd_llc > sd_asym_cpucapacity ever being an
> actual thing - I don't believe it makes much sense, but that's not stopping
> anyone.
> 
> AFAIA we (Arm) *currently* don't allow that with big.LITTLE or DynamIQ, nor
> do I think it can happen with the default scheduler topology where MC is
> the last possible level we can have as sd_llc.
> 
> So it *might* be a safe assumption - and I can still add a SCHED_WARN_ON().

Agreed.

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