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Commit 5cb866ac authored by Chris Redpath's avatar Chris Redpath
Browse files

ANDROID: sched/fair: Fall back to slow path wakeup placement if needed 



Sometimes we can be in a scenario where we are unable to calculate an
energy-aware task placement. This can happen for a few reasons.
1. We might be racing with a hotplug and be unable to access the
   whole-system energy structures
2. The system might be in a condition (due to task size and load)
   whereby there is only one potential place to put a task (when
   we consider avoiding overutilization etc.).
3. There might actually be no non-overutilized sched domain in
   the system where the current task would fit.

We handle case 3 directly in select_task_rq_fair - there is no point
even checking what the energy situation might be since we can already
tell that it is not possible to do an energy-aware placement of this
task. When that is apparent, we null out the energy_sd and fallback to
the regular slow-path wakeup (which is also capacity-aware).

For the other cases, we can't tell we are in such a scenario until we do
at least a little further digging. Allow find_energy_efficient_cpu to
return -1 to indicate that we did not attempt an energy-aware wakeup as
either we couldn't or there was only one potential candidate.

When that happens, we will set new_cpu == -1 and drop again into
find_idlest_cpu to place the task (which is capacity aware).

Change-Id: I65454ea7bba6b85ee0467f28929650e80cba0cab
Signed-off-by: default avatarChris Redpath <chris.redpath@arm.com>
parent c8d50e06

kernel/sched/fair.c

+27 −11
Original line number Diff line number Diff line
@@ -5826,14 +5826,26 @@ static inline bool cpu_in_sg(struct sched_group *sg, int cpu) 
 * select_energy_cpu_idx(): estimate the energy impact of changing the

 * utilization distribution.

 *

 * The eenv parameter specifies the changes: utilisation amount and a pair of

 * possible CPU candidates (the previous CPU and a different target CPU).

 * The eenv parameter specifies the changes: utilization amount and a

 * collection of possible CPU candidates. The number of candidates

 * depends upon the selection algorithm used.

 *

 * If find_best_target was used to select candidate CPUs, there will

 * be at most 3 including prev_cpu. If not, we used a brute force

 * selection which will provide the union of:

 *  * CPUs belonging to the highest sd which is not overutilized

 *  * CPUs the task is allowed to run on

 *  * online CPUs

 *

 * This function returns the index of a CPU candidate specified by the

 * energy_env which corresponds to the most energy_efficient CPU.

 * energy_env which corresponds to the most energy efficient CPU.

 * Thus, 0 (EAS_CPU_PRV) means that non of the CPU candidate is more energy

 * efficient than running on prev_cpu. This is also the value returned in case

 * of abort due to error conditions during the computations.

 * of abort due to error conditions during the computations. The only

 * exception to this if we fail to access the energy model via sd_ea, where

 * we return -1 with the intent of asking the system to use a different

 * wakeup placement algorithm.

 *

 * A value greater than zero means that the most energy efficient CPU is the

 * one represented by eenv->cpu[eenv->next_idx].cpu_id.

 */
@@ -5849,7 +5861,7 @@ static inline int select_energy_cpu_idx(struct energy_env *eenv) 
	sd_cpu = eenv->cpu[EAS_CPU_PRV].cpu_id;

	sd = rcu_dereference(per_cpu(sd_ea, sd_cpu));

	if (!sd)

		return EAS_CPU_PRV;

		return -1;



	cpumask_clear(&eenv->cpus_mask);

	for (cpu_idx = EAS_CPU_PRV; cpu_idx < eenv->max_cpu_count; ++cpu_idx) {
@@ -7088,7 +7100,7 @@ static int find_energy_efficient_cpu(struct sched_domain *sd, 
{

	int use_fbt = sched_feat(FIND_BEST_TARGET);

	int cpu_iter, eas_cpu_idx = EAS_CPU_NXT;

	int energy_cpu = prev_cpu;

	int energy_cpu = -1;

	struct energy_env *eenv;



	if (sysctl_sched_sync_hint_enable && sync) {
@@ -7097,7 +7109,7 @@ static int find_energy_efficient_cpu(struct sched_domain *sd, 
		}

	}



	/* take ownership of our per-cpu data structure */

	/* prepopulate energy diff environment */

	eenv = get_eenv(p, prev_cpu);

	if (eenv->max_cpu_count < 2)

		return energy_cpu;
@@ -7168,8 +7180,8 @@ static int find_energy_efficient_cpu(struct sched_domain *sd, 
	}



	/* find most energy-efficient CPU */

	eas_cpu_idx = select_energy_cpu_idx(eenv);

	energy_cpu = eenv->cpu[eas_cpu_idx].cpu_id;

	energy_cpu = select_energy_cpu_idx(eenv) < 0 ? -1 :

					eenv->cpu[eenv->next_idx].cpu_id;



	return energy_cpu;

}
@@ -7323,7 +7335,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f 
	} else {

		if (energy_sd)

			new_cpu = find_energy_efficient_cpu(energy_sd, p, cpu, prev_cpu, sync);

		else



		/* if we did an energy-aware placement and had no choices available

		 * then fall back to the default find_idlest_cpu choice

		 */

		if (!energy_sd || (energy_sd && new_cpu == -1))

			new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);

	}