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sched/rt: Fix bad task migration for rt tasks

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Commit 95158a89dd ("sched,rt: Use the full cpumask for balancing")
allows find_lock_lowest_rq() to pick a task with migration disabled.
The purpose of the commit is to push the current running task on the
CPU that has the migrate_disable() task away.

However, there is a race which allows a migrate_disable() task to be
migrated. Consider:

  CPU0                                    CPU1
  push_rt_task
    check is_migration_disabled(next_task)

                                          task not running and
                                          migration_disabled == 0

    find_lock_lowest_rq(next_task, rq);
      _double_lock_balance(this_rq, busiest);
        raw_spin_rq_unlock(this_rq);
        double_rq_lock(this_rq, busiest);
          <<wait for busiest rq>>
                                              <wakeup>
                                          task become running
                                          migrate_disable();
                                            <context out>
    deactivate_task(rq, next_task, 0);
    set_task_cpu(next_task, lowest_rq->cpu);
      WARN_ON_ONCE(is_migration_disabled(p));

Fixes: 95158a89dd ("sched,rt: Use the full cpumask for balancing")
Signed-off-by: Schspa Shi <schspa@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Dwaine Gonyier <dgonyier@redhat.com>
This commit is contained in:
Schspa Shi 2022-08-29 01:03:02 +08:00 committed by Peter Zijlstra
parent 223baf9d17
commit feffe5bb27
2 changed files with 5 additions and 0 deletions
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1
kernel/sched/deadline.c
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@ -2246,6 +2246,7 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)
!cpumask_test_cpu(later_rq->cpu, &task->cpus_mask) ||
task_on_cpu(rq, task) ||
!dl_task(task) ||
is_migration_disabled(task) ||
!task_on_rq_queued(task))) {
double_unlock_balance(rq, later_rq);
later_rq = NULL;

4
kernel/sched/rt.c
View File

@ -2000,11 +2000,15 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
* the mean time, task could have
* migrated already or had its affinity changed.
* Also make sure that it wasn't scheduled on its rq.
* It is possible the task was scheduled, set
* "migrate_disabled" and then got preempted, so we must
* check the task migration disable flag here too.
*/
if (unlikely(task_rq(task) != rq ||
!cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
task_on_cpu(rq, task) ||
!rt_task(task) ||
is_migration_disabled(task) ||
!task_on_rq_queued(task))) {
double_unlock_balance(rq, lowest_rq);