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SubjectRe: [patch] Re: scheduler oddity [bug?]
Ingo Molnar wrote:
> * Mike Galbraith <efault@gmx.de> wrote:
>


...
>> OK, I've not seen any problem indications yet, so find patchlet below.
>>
>> However! Balazs has stated that this problem is _not_ present in .git,
>> and that..
>>
>> commit 38736f475071b80b66be28af7b44c854073699cc
>> Author: Gautham R Shenoy <ego@in.ibm.com>
>> Date: Sat Sep 6 14:50:23 2008 +0530
>>
>> ..is what fixed it. Willy Tarreau verified this as being the case on
>> his HW as well. It is present in .git with my HW.
>>
>> I see it as a problem, but it's your call. Dunno if I'd apply it or
>> hold back, given these conflicting reports.
>>
>
> I think we still want it - as the purpose of the overlap metric
> is to measure reality. If preemption causes overlap in execution
> we should not ignore that.
>

I'm sure it's wrong. The only call to dequeue with a non-zero sleep
value is in deactivate_task. All the rest have zero sleep. The section
of code identified by Mike in his patchlet should be moved for purpose
of clarity. It also hilights the symmetry between queue_task and
dequeue_task:

--- sched.c 2009-02-21 09:09:34.000000000 +1030
+++ sched.c.dn 2009-03-09 20:13:51.000000000 +1030
@@ -1647,12 +1647,6 @@

static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
- if (sleep && p->se.last_wakeup) {
- update_avg(&p->se.avg_overlap,
- p->se.sum_exec_runtime - p->se.last_wakeup);
- p->se.last_wakeup = 0;
- }
-
sched_info_dequeued(p);
p->sched_class->dequeue_task(rq, p, sleep);
p->se.on_rq = 0;
@@ -1724,6 +1718,12 @@
if (task_contributes_to_load(p))
rq->nr_uninterruptible++;

+ if (sleep && p->se.last_wakeup) {
+ update_avg(&p->se.avg_overlap,
+ p->se.sum_exec_runtime - p->se.last_wakeup);
+ p->se.last_wakeup = 0;
+ }
+
dequeue_task(rq, p, sleep);
dec_nr_running(rq);
}


Having done that, it makes sense to entirely remove dequeue_task 's
sleep parameter, and replicate all three lines in deactivate_task:

--- sched.c.dn 2009-03-09 20:41:13.000000000 +1030
+++ sched.c.dn2 2009-03-09 20:41:30.000000000 +1030
@@ -1645,10 +1645,10 @@
p->se.on_rq = 1;
}

-static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
+static void dequeue_task(struct rq *rq, struct task_struct *p)
{
sched_info_dequeued(p);
- p->sched_class->dequeue_task(rq, p, sleep);
+ p->sched_class->dequeue_task(rq, p, 0);
p->se.on_rq = 0;
}

@@ -1724,7 +1724,11 @@
p->se.last_wakeup = 0;
}

- dequeue_task(rq, p, sleep);
+ /*dequeue_task(rq, p, sleep);*/
+ sched_info_dequeued(p);
+ p->sched_class->dequeue_task(rq, p, sleep);
+ p->se.on_rq = 0;
+
dec_nr_running(rq);
}

@@ -4848,7 +4852,7 @@
on_rq = p->se.on_rq;
running = task_current(rq, p);
if (on_rq)
- dequeue_task(rq, p, 0);
+ dequeue_task(rq, p);
if (running)
p->sched_class->put_prev_task(rq, p);

@@ -4897,7 +4901,7 @@
}
on_rq = p->se.on_rq;
if (on_rq)
- dequeue_task(rq, p, 0);
+ dequeue_task(rq, p);

p->static_prio = NICE_TO_PRIO(nice);
set_load_weight(p);
@@ -8637,7 +8641,7 @@
on_rq = tsk->se.on_rq;

if (on_rq)
- dequeue_task(rq, tsk, 0);
+ dequeue_task(rq, tsk);
if (unlikely(running))
tsk->sched_class->put_prev_task(rq, tsk);




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