[PATCH] sched/rt : return accurate release rq lock info

[PATCH] sched/rt : return accurate release rq lock info

[Peng Hao]

find_lock_lowest_rq may or not releease rq lock, but it is fuzzy.
If not releasing rq lock, it is unnecessary to re-call
pick_next_oushable_task.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
---
 kernel/sched/rt.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2e2955a..4d7d322 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1719,6 +1719,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 {
 	struct rq *lowest_rq = NULL;
 	int tries;
+	bool release_lock = false;
 	int cpu;
 
 	for (tries = 0; tries < RT_MAX_TRIES; tries++) {
@@ -1741,6 +1742,7 @@ 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)) {
+			release_lock = true;
 			/*
 			 * We had to unlock the run queue. In
 			 * the mean time, task could have
@@ -1768,6 +1770,8 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 		lowest_rq = NULL;
 	}
 
+	if (!lowest_rq && !release_lock)
+		lowest_rq = (void *) -1;
 	return lowest_rq;
 }
 
@@ -1863,6 +1867,9 @@ static int push_rt_task(struct rq *rq)
 		goto retry;
 	}
 
+	if (lowest_rq == (void *) -1)
+		goto out;
+
 	deactivate_task(rq, next_task, 0);
 	set_task_cpu(next_task, lowest_rq->cpu);
 	activate_task(lowest_rq, next_task, 0);
-- 
1.8.3.1

Re: [PATCH] sched/rt : return accurate release rq lock info

[Peter Zijlstra]

On Tue, Oct 02, 2018 at 10:23:41PM +0800, Peng Hao wrote:
> find_lock_lowest_rq may or not releease rq lock, but it is fuzzy.
> If not releasing rq lock, it is unnecessary to re-call
> pick_next_oushable_task.

You forgot to mention how much this matters. That is, why did you take
the effort to write this patch.

> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> ---
>  kernel/sched/rt.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 2e2955a..4d7d322 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1719,6 +1719,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
>  {
>  	struct rq *lowest_rq = NULL;
>  	int tries;
> +	bool release_lock = false;
>  	int cpu;

We generally prefer to keep the variable definitions (reverse) ordered
on line length (reverse-xmas-tree).

>  
>  	for (tries = 0; tries < RT_MAX_TRIES; tries++) {
> @@ -1741,6 +1742,7 @@ 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)) {
> +			release_lock = true;
>  			/*
>  			 * We had to unlock the run queue. In
>  			 * the mean time, task could have
> @@ -1768,6 +1770,8 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
>  		lowest_rq = NULL;
>  	}
>  
> +	if (!lowest_rq && !release_lock)
> +		lowest_rq = (void *) -1;

We have a name for that thing, RETRY_TASK. Funnily it means the exact
opposite of what you did. So maybe use that and invert the logic.

>  	return lowest_rq;
>  }
>  

You also forgot to do the same to the deadline code; or explain why it
doesn't need it.