[PATCH 03/12] sched/deadline: Do not BUG() on a deadline task with no active CPU allowed

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

dl_task_root_domain_cpu() finds the root domain that holds a deadline
task's bandwidth from the first active CPU of the task's mask, and
BUG()s when the mask has none. A mask can have none:

 - a task boosted into the deadline class by priority inheritance keeps
   the affinity of its own policy, which may allow one CPU alone, or a
   cpuset of one. dl_task_needs_bw_move() and set_cpus_allowed_dl()
   take it for a deadline task, dl_task(p) being true, and look the root
   domain up from the mask they are replacing, which allows that CPU,
   inactive by then: the cpuset code updates the masks of a cpuset's
   tasks once the CPU is inactive, and select_fallback_rq() widens the
   mask of a task it pushes off that CPU;
 - with admission control off (sched_rt_runtime_us at -1), a deadline
   task may be pinned to fewer CPUs than its root domain spans, and
   those may all go offline.

Both take the kernel down on the first try, in 8-CPU VMs of v7.4-rc1
with the four deadline fixes of sched/urgent. The priority inheritance
one, which asks for no unusual configuration, through the wakeup of the
boosted owner:

  kernel BUG at kernel/sched/deadline.c:3340!
  CPU: 7 UID: 0 PID: 0 Comm: swapper/7
  RIP: 0010:set_cpus_allowed_dl+0xf0/0x100
  Call Trace:
   <IRQ>
   set_cpus_allowed_force+0x9f/0xf0
   cpuset_cpus_allowed_fallback+0x12b/0x1d0
   select_fallback_rq+0x238/0x340
   try_to_wake_up+0x2a0/0xb70

Fall back to the CPU the task last ran on, which these callers used
before the commit this one fixes. Its root domain may have no active CPU
left: the previous patch has __dl_sub() and __dl_add() take that. Ten
rounds of each case leave no warning with it.

Fixes: b4a61fdc19c5 ("sched/deadline: Fix stale root domain in cpuset bandwidth transfer")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/sched/deadline.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index dadda1a528ec..30a52e42c26d 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3338,13 +3338,19 @@ static void task_woken_dl(struct rq *rq, struct task_struct *p)
  * current cpus_mask (not yet updated to the new mask) to find the
  * actual root domain. Use cpus_mask rather than cpus_ptr, which
  * may be temporarily narrowed by migrate_disable().
+ *
+ * The mask has no active CPU left when the CPUs it allows went offline,
+ * which the affinity of a deadline task allows with admission control
+ * off, and that of a task boosted into the deadline class by priority
+ * inheritance always: fall back to the CPU the task last ran on.
  */
 static unsigned int dl_task_root_domain_cpu(struct task_struct *p)
 {
 	unsigned int cpu;
 
 	cpu = cpumask_first_and(&p->cpus_mask, cpu_active_mask);
-	BUG_ON(cpu >= nr_cpu_ids);
+	if (cpu >= nr_cpu_ids)
+		cpu = task_cpu(p);
 	return cpu;
 }
 
-- 
2.47.3