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

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Date:   Fri, 5 Oct 2018 17:46:32 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Peng Hao <peng.hao2@....com.cn>
Cc:     mingo@...hat.com, peterz@...radead.org, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com
Subject: Re: [PATCH v3] sched/rt : return accurate release rq lock info

Hi Peng,

On Sat, Oct 06, 2018 at 06:22:11AM +0800, Peng Hao wrote:
> find_lock_lowest_rq may or not releease rq lock when return
> lowest_rq=NULL, but it is fuzzy.
> If not releasing rq lock, it is unnecessary to re-call
> pick_next_pushable_task.

IIRC, deadline.c uses a similar pattern (c.f., find_lock_later_rq() and
pick_next_pushable_dl_task()): should it be considered for this change?

  Andrea


> When CONFIG_PREEMPT=n, not releasing rq lock and return
> lowest_rq=null frequently happens in a simple test case:
> Four different rt priority tasks run on limited two cpus.
> Thanks for Steven Rostedt's advice.
> 
> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
> ---
>  kernel/sched/rt.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> 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;
>  			}
>  		}
> @@ -1830,7 +1830,9 @@ static int push_rt_task(struct rq *rq)
>  
>  	/* find_lock_lowest_rq locks the rq if found */
>  	lowest_rq = find_lock_lowest_rq(next_task, rq);
> -	if (!lowest_rq) {
> +	if (!lowest_rq)
> +		goto out;
> +	if (lowest_rq == RETRY_TASK) {
>  		struct task_struct *task;
>  		/*
>  		 * find_lock_lowest_rq releases rq->lock
> -- 
> 1.8.3.1
>