Re: [PATCH] Fix migration of SCHED_DEADLINE tasks

Juri Lelli
2015-10-15T16:40:26Z

Hi Luca,

On 15/10/15 12:09, Luca Abeni wrote:
> Commit 9d5142624256 ("sched/deadline: Reduce rq lock contention by
> eliminating locking of non-feasible target") broke select_task_rq_dl()
> and find_lock_later_rq(), because it introduced a comparison between
> the local task's deadline and dl.earliest_dl.curr of the remote queue.
> However, if the remote runqueue does not contain any SCHED_DEADLINE
> task its earliest_dl.curr is 0 (always smaller than the deadline of
> the local task) and the remote runqueue is not selected for pushing.
> As a result, if an application creates multiple SCHED_DEADLINE threads,
> they will never be pushed to runqueues that do not already contain
> SCHED_DEADLINE tasks.
> This patches fixes the issue by checking if dl.earliest_dl.curr == 0.
>
> Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
> ---
> kernel/sched/deadline.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fc8f010..0d86d60 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1066,8 +1066,9 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
> int target = find_later_rq(p);
>
> if (target != -1 &&
> - dl_time_before(p->dl.deadline,
> - cpu_rq(target)->dl.earliest_dl.curr))
> + (dl_time_before(p->dl.deadline,
> + cpu_rq(target)->dl.earliest_dl.curr) ||
> + (cpu_rq(target)->dl.earliest_dl.curr == 0)))

Can't we actually use dl.dl_nr_running here and below, so
that we won't incur any wraparound problem?

Thanks,

- Juri

> cpu = target;
> }
> rcu_read_unlock();
> @@ -1417,7 +1418,8 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)
>
> later_rq = cpu_rq(cpu);
>
> - if (!dl_time_before(task->dl.deadline,
> + if (later_rq->dl.earliest_dl.curr &&
> + !dl_time_before(task->dl.deadline,
> later_rq->dl.earliest_dl.curr)) {
> /*
> * Target rq has tasks of equal or earlier deadline,
>