sched/core: Provide a pointer to the valid CPU mask

In commit:

  4b53a3412d66 ("sched/core: Remove the tsk_nr_cpus_allowed() wrapper")

the tsk_nr_cpus_allowed() wrapper was removed. There was not
much difference in !RT but in RT we used this to implement
migrate_disable(). Within a migrate_disable() section the CPU mask is
restricted to single CPU while the "normal" CPU mask remains untouched.

As an alternative implementation Ingo suggested to use:

	struct task_struct {
		const cpumask_t		*cpus_ptr;
		cpumask_t		cpus_mask;
        };
with
	t->cpus_ptr = &t->cpus_mask;

In -RT we then can switch the cpus_ptr to:

	t->cpus_ptr = &cpumask_of(task_cpu(p));

in a migration disabled region. The rules are simple:

 - Code that 'uses' ->cpus_allowed would use the pointer.
 - Code that 'modifies' ->cpus_allowed would use the direct mask.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190423142636.14347-1-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 50316455ea66..d57fb2f8ae67 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -124,14 +124,14 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
 	const struct sched_dl_entity *dl_se = &p->dl;
 
 	if (later_mask &&
-	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
+	    cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) {
 		return 1;
 	} else {
 		int best_cpu = cpudl_maximum(cp);
 
 		WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
 
-		if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
+		if (cpumask_test_cpu(best_cpu, p->cpus_ptr) &&
 		    dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
 			if (later_mask)
 				cpumask_set_cpu(best_cpu, later_mask);