[PATCH 11/20] sched: Handle CPU isolation on last resort fallback rq selection

[Frederic Weisbecker <frederic@kernel.org>]

When a kthread or any other task has an affinity mask that is fully
offline or unallowed, the scheduler reaffines the task to all possible
CPUs as a last resort.

This default decision doesn't mix up very well with nohz_full CPUs that
are part of the possible cpumask but don't want to be disturbed by
unbound kthreads or even detached pinned user tasks.

Make the fallback affinity setting aware of nohz_full. This applies to
all architectures supporting nohz_full except arm32. However this
architecture that overrides the task possible mask is unlikely to be
willing to integrate new development.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/sched/core.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 43e453ab7e20..d4b759c1cbf1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3421,6 +3421,21 @@ void kick_process(struct task_struct *p)
 }
 EXPORT_SYMBOL_GPL(kick_process);
 
+static const struct cpumask *task_cpu_fallback_mask(struct task_struct *t)
+{
+	const struct cpumask *mask;
+
+	mask = task_cpu_possible_mask(p);
+	/*
+	 * Architectures that overrides the task possible mask
+	 * must handle CPU isolation.
+	 */
+	if (mask != cpu_possible_mask)
+		return mask;
+	else
+		return housekeeping_cpumask(HK_TYPE_TICK);
+}
+
 /*
  * ->cpus_ptr is protected by both rq->lock and p->pi_lock
  *
@@ -3489,7 +3504,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 			 *
 			 * More yuck to audit.
 			 */
-			do_set_cpus_allowed(p, task_cpu_possible_mask(p));
+			do_set_cpus_allowed(p, task_cpu_fallback_mask(p));
 			state = fail;
 			break;
 		case fail:
-- 
2.46.0