linux-kernel - Re: [RFC PATCH 1/3] sched/core: Provide a method to check if a task is PI-boosted.

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Message-ID: <ZUNsSqHiYu8WTHBb@localhost.localdomain>
Date:   Thu, 2 Nov 2023 10:30:50 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        John Stultz <jstultz@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC PATCH 1/3] sched/core: Provide a method to check if a task
 is PI-boosted.

Hi,

Apologies for noticing only now, but I believe this is still part of the
6.6-rt patches and I've got the below question to ask.

On 04/08/23 13:30, Sebastian Andrzej Siewior wrote:
> Provide a method to check if a task inherited the priority from another
> task. This happens if a task owns a lock which is requested by a task
> with higher priority. This can be used as a hint to add a preemption
> point to the critical section.
> 
> Provide a function which reports true if the task is PI-boosted.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  include/linux/sched.h |  1 +
>  kernel/sched/core.c   | 15 +++++++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 609bde814cb06..77fd274133750 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1887,6 +1887,7 @@ static inline int dl_task_check_affinity(struct task_struct *p, const struct cpu
>  }
>  #endif
>  
> +extern bool task_is_pi_boosted(const struct task_struct *p);
>  extern int yield_to(struct task_struct *p, bool preempt);
>  extern void set_user_nice(struct task_struct *p, long nice);
>  extern int task_prio(const struct task_struct *p);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c52c2eba7c739..132f06522efa0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8886,6 +8886,21 @@ static inline void preempt_dynamic_init(void) { }
>  
>  #endif /* #ifdef CONFIG_PREEMPT_DYNAMIC */
>  
> +/*
> + * task_is_pi_boosted - Check if task has been PI boosted.
> + * @p:	Task to check.
> + *
> + * Return true if task is subject to priority inheritance.
> + */
> +bool task_is_pi_boosted(const struct task_struct *p)
> +{
> +	int prio = p->prio;
> +
> +	if (!rt_prio(prio))
> +		return false;
> +	return prio != p->normal_prio;

Does this need to also take DEADLINE tasks into consideration? We don't
change priority when they are boosted, only pi_se changes (please check
is_dl_boosted()).

Thanks,
Juri