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

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Message-ID: <20181002080134.GR3439@hirez.programming.kicks-ass.net>
Date:   Tue, 2 Oct 2018 10:01:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peng Hao <peng.hao2@....com.cn>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH]  sched/rt : return accurate release rq lock info

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@....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.