linux-kernel - Re: [PATCH] kernel/sched/core: adjust rt_priority accordingly when prio is changed

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Message-ID: <20230201105603.1d377866@gandalf.local.home>
Date:   Wed, 1 Feb 2023 10:56:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Song Chen <chensong_2000@....cn>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/sched/core: adjust rt_priority accordingly when
 prio is changed

On Wed,  1 Feb 2023 18:01:20 +0800
Song Chen <chensong_2000@....cn> wrote:

> When a high priority process is acquiring a rtmutex which is held by a
> low priority process, the latter's priority will be boosted up by calling
> rt_mutex_setprio->__setscheduler_prio.
> 
> However, p->prio is changed but p->rt_priority is not, as a result, the
> equation between prio and rt_priority is broken, which is:
> 
> 	prio = MAX_RT_PRIO - 1 - rt_priority
> 
> It's confusing to the user when it calls sched_getparam, which only
> returns rt_priority.

If it is boosted, then that's an internal implementation and not the real
priority of the task. It should not be exposed to a user interface. In
fact, there's discussion of implementing a "proxy" algorithm which will
make what the "priority" of a task is even more complicated when acquiring
mutexes.

> 
> This patch addresses this issue by adjusting rt_priority according to
> the new value of prio, what's more, it also returns normal_prio for
> CFS processes instead of just a zero.

The comment above sched_getparam() is:

/**
 * sys_sched_getparam - get the RT priority of a thread
 * @pid: the pid in question.
 * @param: structure containing the RT priority.
 *
 * Return: On success, 0 and the RT priority is in @param. Otherwise, an error
 * code.
 */

So returning the nice value is incorrect. If anything, perhaps it should
return -EINVAL if the task is not an RT task?

-- Steve

> 
> Signed-off-by: Song Chen <chensong_2000@....cn>
> ---