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SubjectRe: [PATCH] sched/rt.c: pick and check task if double_lock_balance() unlock the rq
polite ping and +cc, thanks!

On 2017/9/11 14:51, Zhou Chengming wrote:
> 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


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