kernel: 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 (pid == 1) must be
explicitly excluded from this so that all processes forked from init have a
sane default CPU affinity mask.

Signed-off-by: Sultan Alsawaf <sultanxda@gmail.com>

kernel/sched/core.c

@@ -1082,6 +1082,10 @@ adjust_cpumask(const struct task_struct *p,
 	/* Force all performance-critical kthreads onto the big cluster */
 	if (p->flags & PF_PERF_CRITICAL)
 		return cpu_perf_mask;
+	/* Force all trivial, unbound kthreads onto the little cluster */
+	if (p->flags & PF_KTHREAD && p->pid != 1 &&
+		cpumask_equal(req_mask, cpu_all_mask))
+		return cpu_lp_mask;
 
 	return orig_mask;
 }