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Date: Wed, 11 Jun 2008 00:58:30 +0200
From: Dmitry Adamushko <dmitry.adamushko@...il.com>
To: Gregory Haskins <ghaskins@...ell.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
linux-kernel@...r.kernel.org
Subject: [sched-devel, patch-rfc] rework of "prioritize non-migratable
tasks over migratable ones"
Hi Gregory,
regarding this commit: 45c01e824991b2dd0a332e19efc4901acb31209f
I think we can do it simpler. Please take a look at the patch below.
Instead of having 2 separate arrays (which is + ~800 bytes on x86_32 and twice so on x86_64),
let's add "exclusive" (the ones that are bound to this CPU) tasks to the head of the queue
and "shared" ones -- to the end.
In case of a few newly woken up "exclusive" tasks, they are 'stacked' (not queued as now), meaning that
a task {i+1} is being placed in front of the previously woken up task {i}. But I don't think that
this behavior may cause any realistic problems.
There are a couple of changes on top of this one.
(1) in check_preempt_curr_rt()
I don't think there is a need for the "pick_next_rt_entity(rq, &rq->rt) != &rq->curr->rt" check.
enqueue_task_rt(p) and check_preempt_curr_rt() are always called one after another with rq->lock being held so
the following check "p->rt.nr_cpus_allowed == 1 && rq->curr->rt.nr_cpus_allowed != 1"
should be enough (well, just its left part) to guarantee that 'p' has been queued in front of the 'curr'.
(2) in set_cpus_allowed_rt()
I don't thinks there is a need for requeue_task_rt() here.
Perhaps, the only case when 'requeue' (+ reschedule) might be useful is as follows:
i) weight == 1 && cpu_isset(task_cpu(p), *new_mask)
i.e. a task is being bound to this CPU);
ii) 'p' != rq->curr
but here, 'p' has already been on this CPU for a while and was not migrated. i.e. it's possible that 'rq->curr'
would not have high chances to be migrated right at this particular moment (although, has chance in a bit longer term),
should we allow it to be preempted.
Anyway, I think we should not perhaps make it more complex trying to address some rare corner cases.
For instance, that's why a single queue approach would be preferable. Unless I'm missing something obvious,
this approach gives us similar functionality at lower cost.
Verified only compilation-wise.
(Almost)-Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>
---
diff --git a/kernel/sched.c b/kernel/sched.c
index 268a850..e1c382d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -155,8 +155,7 @@ static inline int task_has_rt_policy(struct task_struct *p)
*/
struct rt_prio_array {
DECLARE_BITMAP(bitmap, MAX_RT_PRIO+1); /* include 1 bit for delimiter */
- struct list_head xqueue[MAX_RT_PRIO]; /* exclusive queue */
- struct list_head squeue[MAX_RT_PRIO]; /* shared queue */
+ struct list_head queue[MAX_RT_PRIO];
};
struct rt_bandwidth {
@@ -7661,8 +7660,7 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
array = &rt_rq->active;
for (i = 0; i < MAX_RT_PRIO; i++) {
- INIT_LIST_HEAD(array->xqueue + i);
- INIT_LIST_HEAD(array->squeue + i);
+ INIT_LIST_HEAD(array->queue + i);
__clear_bit(i, array->bitmap);
}
/* delimiter for bitsearch: */
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 49a225e..2437504 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -576,16 +576,15 @@ static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
struct rt_prio_array *array = &rt_rq->active;
struct rt_rq *group_rq = group_rt_rq(rt_se);
+ struct list_head *queue = array->queue + rt_se_prio(rt_se);
if (group_rq && rt_rq_throttled(group_rq))
return;
if (rt_se->nr_cpus_allowed == 1)
- list_add_tail(&rt_se->run_list,
- array->xqueue + rt_se_prio(rt_se));
+ list_add(&rt_se->run_list, queue);
else
- list_add_tail(&rt_se->run_list,
- array->squeue + rt_se_prio(rt_se));
+ list_add_tail(&rt_se->run_list, queue);
__set_bit(rt_se_prio(rt_se), array->bitmap);
@@ -598,8 +597,7 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
struct rt_prio_array *array = &rt_rq->active;
list_del_init(&rt_se->run_list);
- if (list_empty(array->squeue + rt_se_prio(rt_se))
- && list_empty(array->xqueue + rt_se_prio(rt_se)))
+ if (list_empty(array->queue + rt_se_prio(rt_se)))
__clear_bit(rt_se_prio(rt_se), array->bitmap);
dec_rt_tasks(rt_se, rt_rq);
@@ -666,11 +664,6 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
/*
* Put task to the end of the run list without the overhead of dequeue
* followed by enqueue.
- *
- * Note: We always enqueue the task to the shared-queue, regardless of its
- * previous position w.r.t. exclusive vs shared. This is so that exclusive RR
- * tasks fairly round-robin with all tasks on the runqueue, not just other
- * exclusive tasks.
*/
static
void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
@@ -678,7 +671,7 @@ void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
struct rt_prio_array *array = &rt_rq->active;
list_del_init(&rt_se->run_list);
- list_add_tail(&rt_se->run_list, array->squeue + rt_se_prio(rt_se));
+ list_add_tail(&rt_se->run_list, array->queue + rt_se_prio(rt_se));
}
static void requeue_task_rt(struct rq *rq, struct task_struct *p)
@@ -736,9 +729,6 @@ static int select_task_rq_rt(struct task_struct *p, int sync)
}
#endif /* CONFIG_SMP */
-static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
- struct rt_rq *rt_rq);
-
/*
* Preempt the current task with a newly woken task if needed:
*/
@@ -764,8 +754,7 @@ static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
*/
if((p->prio == rq->curr->prio)
&& p->rt.nr_cpus_allowed == 1
- && rq->curr->rt.nr_cpus_allowed != 1
- && pick_next_rt_entity(rq, &rq->rt) != &rq->curr->rt) {
+ && rq->curr->rt.nr_cpus_allowed != 1) {
cpumask_t mask;
if (cpupri_find(&rq->rd->cpupri, rq->curr, &mask))
@@ -789,15 +778,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
idx = sched_find_first_bit(array->bitmap);
BUG_ON(idx >= MAX_RT_PRIO);
- queue = array->xqueue + idx;
- if (!list_empty(queue))
- next = list_entry(queue->next, struct sched_rt_entity,
- run_list);
- else {
- queue = array->squeue + idx;
- next = list_entry(queue->next, struct sched_rt_entity,
- run_list);
- }
+ queue = array->queue + idx;
+ next = list_entry(queue->next, struct sched_rt_entity, run_list);
return next;
}
@@ -867,7 +849,7 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
continue;
if (next && next->prio < idx)
continue;
- list_for_each_entry(rt_se, array->squeue + idx, run_list) {
+ list_for_each_entry(rt_se, array->queue + idx, run_list) {
struct task_struct *p = rt_task_of(rt_se);
if (pick_rt_task(rq, p, cpu)) {
next = p;
@@ -1249,14 +1231,6 @@ static void set_cpus_allowed_rt(struct task_struct *p,
}
update_rt_migration(rq);
-
- if (unlikely(weight == 1 || p->rt.nr_cpus_allowed == 1))
- /*
- * If either the new or old weight is a "1", we need
- * to requeue to properly move between shared and
- * exclusive queues.
- */
- requeue_task_rt(rq, p);
}
p->cpus_allowed = *new_mask;
---
--
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