sched/rt: Make RT capacity-aware

Capacity Awareness refers to the fact that on heterogeneous systems
(like Arm big.LITTLE), the capacity of the CPUs is not uniform, hence
when placing tasks we need to be aware of this difference of CPU
capacities.

In such scenarios we want to ensure that the selected CPU has enough
capacity to meet the requirement of the running task. Enough capacity
means here that capacity_orig_of(cpu) >= task.requirement.

The definition of task.requirement is dependent on the scheduling class.

For CFS, utilization is used to select a CPU that has >= capacity value
than the cfs_task.util.

	capacity_orig_of(cpu) >= cfs_task.util

DL isn't capacity aware at the moment but can make use of the bandwidth
reservation to implement that in a similar manner CFS uses utilization.
The following patchset implements that:

https://lore.kernel.org/lkml/20190506044836.2914-1-luca.abeni@santannapisa.it/

	capacity_orig_of(cpu)/SCHED_CAPACITY >= dl_deadline/dl_runtime

For RT we don't have a per task utilization signal and we lack any
information in general about what performance requirement the RT task
needs. But with the introduction of uclamp, RT tasks can now control
that by setting uclamp_min to guarantee a minimum performance point.

ATM the uclamp value are only used for frequency selection; but on
heterogeneous systems this is not enough and we need to ensure that the
capacity of the CPU is >= uclamp_min. Which is what implemented here.

	capacity_orig_of(cpu) >= rt_task.uclamp_min

Note that by default uclamp.min is 1024, which means that RT tasks will
always be biased towards the big CPUs, which make for a better more
predictable behavior for the default case.

Must stress that the bias acts as a hint rather than a definite
placement strategy. For example, if all big cores are busy executing
other RT tasks we can't guarantee that a new RT task will be placed
there.

On non-heterogeneous systems the original behavior of RT should be
retained. Similarly if uclamp is not selected in the config.

[ mingo: Minor edits to comments. ]

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191009104611.15363-1-qais.yousef@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 23 insertions, 2 deletions

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index b7abca987d94..1a2719e1350a 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -46,6 +46,8 @@ static int convert_prio(int prio)
  * @cp: The cpupri context
  * @p: The task
  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
+ * @fitness_fn: A pointer to a function to do custom checks whether the CPU
+ *              fits a specific criteria so that we only return those CPUs.
  *
  * Note: This function returns the recommended CPUs as calculated during the
  * current invocation.  By the time the call returns, the CPUs may have in
@@ -57,7 +59,8 @@ static int convert_prio(int prio)
  * Return: (int)bool - CPUs were found
  */
 int cpupri_find(struct cpupri *cp, struct task_struct *p,
-		struct cpumask *lowest_mask)
+		struct cpumask *lowest_mask,
+		bool (*fitness_fn)(struct task_struct *p, int cpu))
 {
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
@@ -98,6 +101,8 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 			continue;
 
 		if (lowest_mask) {
+			int cpu;
+
 			cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
 
 			/*
@@ -108,7 +113,23 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 			 * condition, simply act as though we never hit this
 			 * priority level and continue on.
 			 */
-			if (cpumask_any(lowest_mask) >= nr_cpu_ids)
+			if (cpumask_empty(lowest_mask))
+				continue;
+
+			if (!fitness_fn)
+				return 1;
+
+			/* Ensure the capacity of the CPUs fit the task */
+			for_each_cpu(cpu, lowest_mask) {
+				if (!fitness_fn(p, cpu))
+					cpumask_clear_cpu(cpu, lowest_mask);
+			}
+
+			/*
+			 * If no CPU at the current priority can fit the task
+			 * continue looking
+			 */
+			if (cpumask_empty(lowest_mask))
 				continue;
 		}