linux-kernel - Re: [PATCH v3] sched/rt : return accurate release rq lock info

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Date:   Mon, 15 Oct 2018 11:42:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Peng Hao <peng.hao2@....com.cn>, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/rt : return accurate release rq lock info

On Mon, 15 Oct 2018 11:20:32 +0200
Peter Zijlstra <peterz@...radead.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