linux-kernel - Re: [PATCH v8 5/9] sched/fair: Take into account latency priority at wakeup

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Message-ID: <Y3JqygejPxrtTFgP@google.com>
Date:   Mon, 14 Nov 2022 17:20:26 +0100
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qyousef@...alina.io, chris.hyser@...cle.com,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org
Subject: Re: [PATCH v8 5/9] sched/fair: Take into account latency priority at
 wakeup

Hi Vincent,

On 10-Nov 18:50, Vincent Guittot wrote:

[...]

> diff --git a/init/init_task.c b/init/init_task.c
> index 7dd71dd2d261..b8ddf403bc62 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -78,7 +78,7 @@ struct task_struct init_task
>  	.prio		= MAX_PRIO - 20,
>  	.static_prio	= MAX_PRIO - 20,
>  	.normal_prio	= MAX_PRIO - 20,
> -	.latency_nice	= DEFAULT_LATENCY_NICE,
> +	.latency_prio	= NICE_WIDTH - 20,
                    ^^^^^^^^^^

For robustness/consistency, shoudln't this be LATENCY_NICE_WIDTH?

[...]

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b2accc9da4fe..caf54e54a74f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1284,6 +1284,16 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>  	}
>  }
>  
> +static void set_latency_offset(struct task_struct *p)
> +{
> +	long weight = sched_latency_to_weight[p->latency_prio];
> +	s64 offset;
> +
> +	offset = weight * get_sched_latency(false);
                      ^^^^^^^^^^^^^^^^^^^^^^^^
As per my comment in patch 1, we almost always (but one time) call this with
"false" and that's not returning the sysctl_sched_latency but a possibly
discounted value in case of feat(GENTLE_FAIR_SLEEPERS).

Just to avoid confusion (this could be not the sched_latency) and to better
document the code, what about using a accessor define something like e.g.

   #define max_wakeup_latency get_wakeup_latency(false)

?

[...]

Best,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi