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Message-ID: <20151119093810.GA5828@pablo>
Date: Thu, 19 Nov 2015 09:38:10 +0000
From: Juri Lelli <juri.lelli@....com>
To: Wanpeng Li <wanpeng.li@...mail.com>
Cc: Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/deadline: fix earliest_dl.next is not the
pushable candidate
Hi,
On 10/11/15 20:33, Wanpeng Li wrote:
> earliest_dl.next is used to cache the next earliest ready task
> which also is pushable in order to be a candidate of pushable
> tasks during pull algorithm. If the earliest_dl.next deadline
> of the sr_rq is earlier than the earliest_dl.curr deadline of
> current rq, the task from the sr_rq can be pulled. However,
> current implementation logic of earliest_dl.next just guarantee
> it is the next earliest ready task instead of the next pushable
> earliest task, which will result in hold both rqs' lock and find
> nothing to preempt our current since the affinity in pull algorithm.
> 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 fix it by updating earliest_dl.next when pushable dl
> task is enqueued/dequeued the same way as rt class.
>
I'd slightly modify subject and changelog as something like:
sched/deadline: fix earliest_dl.next logic
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.
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v1 -> v2:
> * fix potential NULL pointer dereference
>
> kernel/sched/deadline.c | 61 ++++++++-----------------------------------------
> 1 file changed, 10 insertions(+), 51 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 142df26..b18c116 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -87,6 +87,8 @@ void init_dl_rq(struct dl_rq *dl_rq)
>
> #ifdef CONFIG_SMP
>
> +static struct task_struct *pick_next_pushable_dl_task(struct rq *rq);
> +
> static inline int dl_overloaded(struct rq *rq)
> {
> return atomic_read(&rq->rd->dlo_count);
> @@ -181,11 +183,15 @@ static void enqueue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>
> rb_link_node(&p->pushable_dl_tasks, parent, link);
> rb_insert_color(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> +
> + if (dl_time_before(p->dl.deadline, dl_rq->earliest_dl.next))
> + dl_rq->earliest_dl.next = p->dl.deadline;
> }
>
> static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
> {
> struct dl_rq *dl_rq = &rq->dl;
> + struct task_struct *next_pushable;
>
> if (RB_EMPTY_NODE(&p->pushable_dl_tasks))
> return;
> @@ -199,6 +205,10 @@ static void dequeue_pushable_dl_task(struct rq *rq, struct task_struct *p)
>
> rb_erase(&p->pushable_dl_tasks, &dl_rq->pushable_dl_tasks_root);
> RB_CLEAR_NODE(&p->pushable_dl_tasks);
> +
> + next_pushable = pick_next_pushable_dl_task(rq);
> + if (next_pushable)
> + dl_rq->earliest_dl.next = next_pushable->dl.deadline;
We should reset this to 0 if !next_pushable.
Thanks,
- Juri
> }
>
> static inline int has_pushable_dl_tasks(struct rq *rq)
> @@ -775,42 +785,14 @@ 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);
> }
> }
>
> @@ -832,7 +814,6 @@ 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);
> }
> }
> @@ -1267,28 +1248,6 @@ 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:
> --
> 1.9.1
>
--
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