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Date: Thu, 3 Dec 2015 12:15:02 +0100
From: Luca Abeni <luca.abeni@...tn.it>
To: Wanpeng Li <kernellwp@...il.com>, Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org,
Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v6] sched/deadline: fix earliest_dl.next logic
On 12/03/2015 10:42 AM, Wanpeng Li wrote:
> earliest_dl.next should cache deadline of the earliest ready task that
> is also enqueued in the pushable rbtree, as pull algorithm uses this
> information to find candidates for migration: if the earliest_dl.next
> deadline of source rq is earlier than the earliest_dl.curr deadline of
> destination rq, the task from the source rq can be pulled.
>
> However, current implementation only guarantees that earliest_dl.next is
> the deadline of the next ready task instead of the next pushable task;
> which will result in potentially holding both rqs' lock and find nothing
> to migrate because of affinity constraints. In addition, current logic
> doesn't update the next candidate for pushing in pick_next_task_dl(),
> even if the running task is never eligible.
>
> This patch fixes both problems by updating earliest_dl.next when
> pushable dl task is enqueued/dequeued, similar to what we already do for
> RT.
I just re-ran some tests with this version of the patch, and
it still looks ok.
Luca
>
> Tested-by: Luca Abeni <luca.abeni@...tn.it>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v5 -> v6:
> * take advantage of next_node
> v4 -> v5:
> * remove useless pick_next_earliest_dl_task declare
> v3 -> v4:
> * move earliest_dl.next caculation under if (leftmost)
> * don't reset dl_rq->earliest_dl.next
> * just checking and eventually using the updated leftmost in
> dequeue_pushable_dl_task()
> v2 -> v3:
> * reset dl_rq->earliest_dl.next to 0 if !next_pushable
> v1 -> v2:
> * fix potential NULL pointer dereference
>
> kernel/sched/deadline.c | 58 +++++--------------------------------------------
> 1 file changed, 6 insertions(+), 52 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 8b0a15e..a35e24a 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -176,8 +176,10 @@ static void enqueue_pushable_dl_task(struct rq *rq, struct task_struct *p)
> }
> }
>
> - if (leftmost)
> + if (leftmost) {
> dl_rq->pushable_dl_tasks_leftmost = &p->pushable_dl_tasks;
> + dl_rq->earliest_dl.next = p->dl.deadline;
> + }
>
> rb_link_node(&p->pushable_dl_tasks, parent, link);
> rb_insert_color(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> @@ -195,6 +197,9 @@ static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>
> next_node = rb_next(&p->pushable_dl_tasks);
> dl_rq->pushable_dl_tasks_leftmost = next_node;
> + if (next_node)
> + dl_rq->earliest_dl.next = rb_entry(next_node,
> + struct task_struct, pushable_dl_tasks)->dl.deadline;
> }
>
> rb_erase(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> @@ -782,42 +787,14 @@ static void update_curr_dl(struct rq *rq)
>
> #ifdef CONFIG_SMP
>
> -static struct task_struct *pick_next_earliest_dl_task(struct rq *rq, int cpu);
> -
> -static inline u64 next_deadline(struct rq *rq)
> -{
> - struct task_struct *next = pick_next_earliest_dl_task(rq, rq->cpu);
> -
> - if (next && dl_prio(next->prio))
> - return next->dl.deadline;
> - else
> - return 0;
> -}
> -
> static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
> {
> struct rq *rq = rq_of_dl_rq(dl_rq);
>
> if (dl_rq->earliest_dl.curr == 0 ||
> dl_time_before(deadline, dl_rq->earliest_dl.curr)) {
> - /*
> - * If the dl_rq had no -deadline tasks, or if the new task
> - * has shorter deadline than the current one on dl_rq, we
> - * know that the previous earliest becomes our next earliest,
> - * as the new task becomes the earliest itself.
> - */
> - dl_rq->earliest_dl.next = dl_rq->earliest_dl.curr;
> dl_rq->earliest_dl.curr = deadline;
> cpudl_set(&rq->rd->cpudl, rq->cpu, deadline, 1);
> - } else if (dl_rq->earliest_dl.next == 0 ||
> - dl_time_before(deadline, dl_rq->earliest_dl.next)) {
> - /*
> - * On the other hand, if the new -deadline task has a
> - * a later deadline than the earliest one on dl_rq, but
> - * it is earlier than the next (if any), we must
> - * recompute the next-earliest.
> - */
> - dl_rq->earliest_dl.next = next_deadline(rq);
> }
> }
>
> @@ -839,7 +816,6 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
>
> entry = rb_entry(leftmost, struct sched_dl_entity, rb_node);
> dl_rq->earliest_dl.curr = entry->deadline;
> - dl_rq->earliest_dl.next = next_deadline(rq);
> cpudl_set(&rq->rd->cpudl, rq->cpu, entry->deadline, 1);
> }
> }
> @@ -1274,28 +1250,6 @@ static int pick_dl_task(struct rq *rq, struct task_struct *p, int cpu)
> return 0;
> }
>
> -/* Returns the second earliest -deadline task, NULL otherwise */
> -static struct task_struct *pick_next_earliest_dl_task(struct rq *rq, int cpu)
> -{
> - struct rb_node *next_node = rq->dl.rb_leftmost;
> - struct sched_dl_entity *dl_se;
> - struct task_struct *p = NULL;
> -
> -next_node:
> - next_node = rb_next(next_node);
> - if (next_node) {
> - dl_se = rb_entry(next_node, struct sched_dl_entity, rb_node);
> - p = dl_task_of(dl_se);
> -
> - if (pick_dl_task(rq, p, cpu))
> - return p;
> -
> - goto next_node;
> - }
> -
> - return NULL;
> -}
> -
> /*
> * Return the earliest pushable rq's task, which is suitable to be executed
> * on the CPU, NULL otherwise:
>
--
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