Re: [RFC PATCH v3 09/10] sched/fair: Select an energy-efficient CPU on task wake-up

[Pavan Kondeti]

On Tue, Jun 19, 2018 at 08:57:23AM +0100, Quentin Perret wrote:
> Hi Pavan,
> 
> On Tuesday 19 Jun 2018 at 10:36:01 (+0530), Pavan Kondeti wrote:
> > On Mon, May 21, 2018 at 03:25:04PM +0100, Quentin Perret wrote:
> > 
> > <snip>
> > 
> > > +	if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
> > > +		prev_energy = best_energy = compute_energy(p, prev_cpu);
> > > +	else
> > > +		prev_energy = best_energy = ULONG_MAX;
> > > +
> > > +	for_each_freq_domain(sfd) {
> > > +		unsigned long spare_cap, max_spare_cap = 0;
> > > +		int max_spare_cap_cpu = -1;
> > > +		unsigned long util;
> > > +
> > > +		/* Find the CPU with the max spare cap in the freq. dom. */
> > > +		for_each_cpu_and(cpu, freq_domain_span(sfd), sched_domain_span(sd)) {
> > > +			if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > > +				continue;
> > > +
> > > +			if (cpu == prev_cpu)
> > > +				continue;
> > > +
> > > +			/* Skip CPUs that will be overutilized */
> > > +			util = cpu_util_wake(cpu, p) + task_util;
> > > +			cpu_cap = capacity_of(cpu);
> > > +			if (cpu_cap * 1024 < util * capacity_margin)
> > > +				continue;
> > > +
> > > +			spare_cap = cpu_cap - util;
> > > +			if (spare_cap > max_spare_cap) {
> > > +				max_spare_cap = spare_cap;
> > > +				max_spare_cap_cpu = cpu;
> > > +			}
> > > +		}
> > > +
> > > +		/* Evaluate the energy impact of using this CPU. */
> > > +		if (max_spare_cap_cpu >= 0) {
> > > +			cur_energy = compute_energy(p, max_spare_cap_cpu);
> > > +			if (cur_energy < best_energy) {
> > > +				best_energy = cur_energy;
> > > +				best_energy_cpu = max_spare_cap_cpu;
> > > +			}
> > > +		}
> > > +	}
> > > +
> > > +	/*
> > > +	 * We pick the best CPU only if it saves at least 1.5% of the
> > > +	 * energy used by prev_cpu.
> > > +	 */
> > > +	if ((prev_energy - best_energy) > (prev_energy >> 6))
> > > +		return best_energy_cpu;
> > > +
> > > +	return prev_cpu;
> > > +}
> > 
> > We are comparing the best_energy_cpu against prev_cpu even when prev_cpu
> > can't accommodate the waking task. Is this intentional? Should not we
> > discard the prev_cpu if it can't accommodate the task.
> > 
> > This can potentially make a BIG task run on a lower capacity CPU until
> > load balancer kicks in and corrects the situation.
> 
> We shouldn't enter find_energy_efficient_cpu() in the first place if the
> system is overutilized, so that shouldn't too much of an issue in
> general.
> 

With UTIL_EST enabled, the overutilization condtion may get turned off when
that 1 BIG task goes to sleep. When it wakes up again, we may place it on
the previous CPU due to the above mentioned issue.

It is not just an existing overutilization condition. By placing this task
on the prev_cpu, we may enter overutilization state.

> But yeah, there is one small corner case where prev_cpu is overutilized
> and the system has not been flagged as such yet (when the tasks wakes-up
> shortly before the tick for ex). I think it's possible to cover this case
> by extending the "if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))"
> condition at the top of the function with a check on capacity_margin.
> 

LGTM.

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