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Subject[PATCH] sched/rt.c: pick and check task if double_lock_balance() unlock the rq
Date
push_rt_task() pick the first pushable task and find an eligible
lowest_rq, then double_lock_balance(rq, lowest_rq). So if
double_lock_balance() unlock the rq (when double_lock_balance() return 1),
we have to check if this task is still on the rq.

The problem is that the check conditions are not sufficient:

if (unlikely(task_rq(task) != rq ||
!cpumask_test_cpu(lowest_rq->cpu, &task->cpus_allowed) ||
task_running(rq, task) ||
!rt_task(task) ||
!task_on_rq_queued(task))) {

cpu2 cpu1 cpu0
push_rt_task(rq1)
pick task_A on rq1
find rq0
double_lock_balance(rq1, rq0)
unlock(rq1)
rq1 __schedule
pick task_A run
task_A sleep (dequeued)
lock(rq0)
lock(rq1)
do_above_check(task_A)
task_rq(task_A) == rq1
cpus_allowed unchanged
task_running == false
rt_task(task_A) == true
try_to_wake_up(task_A)
select_cpu = cpu3
enqueue(rq3, task_A)
task_A->on_rq = 1
task_on_rq_queued(task_A)
above_check passed, return rq0
...
migrate task_A from rq1 to rq0

So we can't rely on these checks of task_A to make sure the task_A is
still on the rq1, even though we hold the rq1->lock. This patch will
repick the first pushable task to be sure the task is still on the rq.

Signed-off-by: Zhou Chengming <zhouchengming1@huawei.com>
---
kernel/sched/rt.c | 49 +++++++++++++++++++++++--------------------------
1 file changed, 23 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 45caf93..787b721 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1703,6 +1703,26 @@ static int find_lowest_rq(struct task_struct *task)
return -1;
}

+static struct task_struct *pick_next_pushable_task(struct rq *rq)
+{
+ struct task_struct *p;
+
+ if (!has_pushable_tasks(rq))
+ return NULL;
+
+ p = plist_first_entry(&rq->rt.pushable_tasks,
+ struct task_struct, pushable_tasks);
+
+ BUG_ON(rq->cpu != task_cpu(p));
+ BUG_ON(task_current(rq, p));
+ BUG_ON(p->nr_cpus_allowed <= 1);
+
+ BUG_ON(!task_on_rq_queued(p));
+ BUG_ON(!rt_task(p));
+
+ return p;
+}
+
/* Will lock the rq it finds */
static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
{
@@ -1734,13 +1754,10 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
* We had to unlock the run queue. In
* the mean time, task could have
* migrated already or had its affinity changed.
- * Also make sure that it wasn't scheduled on its rq.
*/
- if (unlikely(task_rq(task) != rq ||
- !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_allowed) ||
- task_running(rq, task) ||
- !rt_task(task) ||
- !task_on_rq_queued(task))) {
+ struct task_struct *next_task = pick_next_pushable_task(rq);
+ if (unlikely(next_task != task ||
+ !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_allowed))) {

double_unlock_balance(rq, lowest_rq);
lowest_rq = NULL;
@@ -1760,26 +1777,6 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
return lowest_rq;
}

-static struct task_struct *pick_next_pushable_task(struct rq *rq)
-{
- struct task_struct *p;
-
- if (!has_pushable_tasks(rq))
- return NULL;
-
- p = plist_first_entry(&rq->rt.pushable_tasks,
- struct task_struct, pushable_tasks);
-
- BUG_ON(rq->cpu != task_cpu(p));
- BUG_ON(task_current(rq, p));
- BUG_ON(p->nr_cpus_allowed <= 1);
-
- BUG_ON(!task_on_rq_queued(p));
- BUG_ON(!rt_task(p));
-
- return p;
-}
-
/*
* If the current CPU has more than one RT task, see if the non
* running task can migrate over to a CPU that is running a task
--
1.8.3.1
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