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SubjectRe: [PATCH v3] sched/rt : return accurate release rq lock info
On Mon, 15 Oct 2018 11:20:32 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > index 2e2955a..be0fc43 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -1754,7 +1754,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
> > !task_on_rq_queued(task))) {
> >
> > double_unlock_balance(rq, lowest_rq);
> > - lowest_rq = NULL;
> > + lowest_rq = RETRY_TASK;
> > break;
> > }
> > }
>
> I'm confused.. should not:
>
> /* try again */
> double_unlock_balance(rq, lowest_rq);
> lowest_rq = NULL;
>
> also return RETRY_TASK? That also is in the double_lock_balance() path
> and will this have had rq->lock() released.

I thought the same thing at first, but this is in the loop path, where
it does everything again. But now looking closer, I think there's a bug
in the original code.

We only do the check if the immediate double_lock_balance() released
the current task rq lock, but we don't take into account if it was
released earlier, which means it could have migrated and we never
noticed!

I believe the code should look like this:

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2e2955a8cf8f..2c9128ce61e2 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1718,6 +1718,7 @@ static int find_lowest_rq(struct task_struct *task)
static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
{
struct rq *lowest_rq = NULL;
+ bool released = false;
int tries;
int cpu;

@@ -1740,7 +1741,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
}

/* if the prio of this runqueue changed, try again */
- if (double_lock_balance(rq, lowest_rq)) {
+ if (double_lock_balance(rq, lowest_rq) || released) {
/*
* We had to unlock the run queue. In
* the mean time, task could have
@@ -1754,7 +1755,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
!task_on_rq_queued(task))) {

double_unlock_balance(rq, lowest_rq);
- lowest_rq = NULL;
+ lowest_rq = RETRY_TASK;
break;
}
}
@@ -1764,10 +1765,15 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
break;

/* try again */
- double_unlock_balance(rq, lowest_rq);
+ if (double_unlock_balance(rq, lowest_rq))
+ released = true;
+
lowest_rq = NULL;
}

+ if (!lowest_rq && released)
+ lowest_rq = RETRY_TASK;
+
return lowest_rq;
}

-- Steve
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