sched/core: Force trivial, unbound kthreads onto low-power CPUs

In order to reduce power consumption, force all non-perf-critical,
unbound kthreads onto the low-power CPUs. Note that init must be
explicitly excluded from this so that all processes forked from init
have a sane default CPU affinity mask.

Change-Id: Ic86928a058d8fb033adc834572cd71248d29e705
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Alex Winkowski <dereference23@outlook.com>
Signed-off-by: Richard Raya <rdxzv.dev@gmail.com>

kernel/sched/core.c

@@ -1073,6 +1073,18 @@ void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_ma
 	p->nr_cpus_allowed = cpumask_weight(new_mask);
 }
 
+static const struct cpumask *
+adjust_cpumask(struct task_struct *p,
+		     const struct cpumask *orig_mask)
+{
+	/* Force all trivial, unbound kthreads onto the little cluster */
+	if (p->flags & PF_KTHREAD && !is_global_init(p) &&
+	    cpumask_equal(orig_mask, cpu_all_mask))
+		return cpu_lp_mask;
+
+	return orig_mask;
+}
+
 void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 {
 	struct rq *rq = task_rq(p);
@@ -1121,6 +1133,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 	int ret = 0;
 	cpumask_t allowed_mask;
 
+	new_mask = adjust_cpumask(p, new_mask);
+
 	rq = task_rq_lock(p, &rf);
 	update_rq_clock(rq);