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Message-ID: <550368F4.5050905@nvidia.com>
Date: Fri, 13 Mar 2015 15:47:16 -0700
From: Sai Gurrappadi <sgurrappadi@...dia.com>
To: Morten Rasmussen <morten.rasmussen@....com>,
"peterz@...radead.org" <peterz@...radead.org>,
"mingo@...hat.com" <mingo@...hat.com>
CC: "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
Dietmar Eggemann <Dietmar.Eggemann@....com>,
"yuyang.du@...el.com" <yuyang.du@...el.com>,
"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
"mturquette@...aro.org" <mturquette@...aro.org>,
"nico@...aro.org" <nico@...aro.org>,
"rjw@...ysocki.net" <rjw@...ysocki.net>,
Juri Lelli <Juri.Lelli@....com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement
On 02/04/2015 10:31 AM, Morten Rasmussen wrote:
> Let available compute capacity and estimated energy impact select
> wake-up target cpu when energy-aware scheduling is enabled.
> energy_aware_wake_cpu() attempts to find group of cpus with sufficient
> compute capacity to accommodate the task and find a cpu with enough spare
> capacity to handle the task within that group. Preference is given to
> cpus with enough spare capacity at the current OPP. Finally, the energy
> impact of the new target and the previous task cpu is compared to select
> the wake-up target cpu.
>
> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
> kernel/sched/fair.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 90 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b371f32..8713310 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5091,6 +5091,92 @@ static int select_idle_sibling(struct task_struct *p, int target)
> done:
> return target;
> }
> +
> +static unsigned long group_max_capacity(struct sched_group *sg)
> +{
> + int max_idx;
> +
> + if (!sg->sge)
> + return 0;
> +
> + max_idx = sg->sge->nr_cap_states-1;
> +
> + return sg->sge->cap_states[max_idx].cap;
> +}
> +
> +static inline unsigned long task_utilization(struct task_struct *p)
> +{
> + return p->se.avg.utilization_avg_contrib;
> +}
> +
> +static int cpu_overutilized(int cpu, struct sched_domain *sd)
> +{
> + return (capacity_orig_of(cpu) * 100) <
> + (get_cpu_usage(cpu) * sd->imbalance_pct);
> +}
> +
> +static int energy_aware_wake_cpu(struct task_struct *p)
> +{
> + struct sched_domain *sd;
> + struct sched_group *sg, *sg_target;
> + int target_max_cap = SCHED_CAPACITY_SCALE;
> + int target_cpu = task_cpu(p);
> + int i;
> +
> + sd = rcu_dereference(per_cpu(sd_ea, task_cpu(p)));
> +
> + if (!sd)
> + return -1;
> +
> + sg = sd->groups;
> + sg_target = sg;
> + /* Find group with sufficient capacity */
> + do {
> + int sg_max_capacity = group_max_capacity(sg);
> +
> + if (sg_max_capacity >= task_utilization(p) &&
> + sg_max_capacity <= target_max_cap) {
> + sg_target = sg;
> + target_max_cap = sg_max_capacity;
> + }
> + } while (sg = sg->next, sg != sd->groups);
If a 'small' task suddenly becomes 'big' i.e close to 100% util, the
above loop would still pick the little/small cluster because
task_utilization(p) is upper-bounded by the arch-invariant capacity of
the little CPU/group right?
Also, this heuristic for determining sg_target is a big little
assumption. I don't think it is necessarily correct to assume that this
is true for all platforms. This heuristic should be derived from the
energy model for the platform instead.
> +
> + /* Find cpu with sufficient capacity */
> + for_each_cpu_and(i, tsk_cpus_allowed(p), sched_group_cpus(sg_target)) {
> + int new_usage = get_cpu_usage(i) + task_utilization(p);
Isn't this double accounting the task's usage in case task_cpu(p)
belongs to sg_target?
> +
> + if (new_usage > capacity_orig_of(i))
> + continue;
> +
> + if (new_usage < capacity_curr_of(i)) {
> + target_cpu = i;
> + if (!cpu_rq(i)->nr_running)
> + break;
> + }
> +
> + /* cpu has capacity at higher OPP, keep it as fallback */
> + if (target_cpu == task_cpu(p))
> + target_cpu = i;
> + }
> +
> + if (target_cpu != task_cpu(p)) {
> + struct energy_env eenv = {
> + .usage_delta = task_utilization(p),
> + .src_cpu = task_cpu(p),
> + .dst_cpu = target_cpu,
> + };
> +
> + /* Not enough spare capacity on previous cpu */
> + if (cpu_overutilized(task_cpu(p), sd))
> + return target_cpu;
> +
> + if (energy_diff(&eenv) >= 0)
> + return task_cpu(p);
> + }
> +
> + return target_cpu;
> +}
> +
> /*
> * select_task_rq_fair: Select target runqueue for the waking task in domains
> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5138,6 +5224,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> prev_cpu = cpu;
>
> if (sd_flag & SD_BALANCE_WAKE) {
> + if (energy_aware()) {
> + new_cpu = energy_aware_wake_cpu(p);
> + goto unlock;
> + }
> new_cpu = select_idle_sibling(p, prev_cpu);
> goto unlock;
> }
>
-Sai
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