linux-kernel - Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

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Date:   Fri, 15 May 2020 13:08:29 +0200
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value


I Qais,
I see we are converging toward the final shape. :)

Function wise code looks ok to me now.

Lemme just point out few more remarks and possible nit-picks.
I guess at the end it's up to you to decide if you wanna follow up with
a v6 and to the maintainers to decide how picky they wanna be.

Otherwise, FWIW, feel free to consider this a LGTM.

Best,
Patrick

On Mon, May 11, 2020 at 17:40:52 +0200, Qais Yousef <qais.yousef@....com> wrote...

[...]

> +static inline void uclamp_sync_util_min_rt_default(struct task_struct *p,
> +						   enum uclamp_id clamp_id)
> +{
> +	unsigned int default_util_min = sysctl_sched_uclamp_util_min_rt_default;
> +	struct uclamp_se *uc_se;
> +
> +	/* Only sync for UCLAMP_MIN and RT tasks */
> +	if (clamp_id != UCLAMP_MIN || !rt_task(p))
> +		return;
> +
> +	uc_se = &p->uclamp_req[UCLAMP_MIN];

I went back to v3 version, where this was done above:

   https://lore.kernel.org/lkml/20200429113255.GA19464@codeaurora.org/
   Message-ID: 20200429113255.GA19464@...eaurora.org

and still I don't see why we want to keep it after this first check?

IMO it's just not required and it makes to code a tiny uglier.

> +
> +	/*
> +	 * Only sync if user didn't override the default request and the sysctl
> +	 * knob has changed.
> +	 */
> +	if (uc_se->user_defined || uc_se->value == default_util_min)
> +		return;
> +

nit-pick: the two comments above are stating the obvious.

> +	uclamp_se_set(uc_se, default_util_min, false);
> +}
> +
>  static inline struct uclamp_se
>  uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
>  {
> @@ -907,8 +949,13 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
>  static inline struct uclamp_se
>  uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
>  {
> -	struct uclamp_se uc_req = uclamp_tg_restrict(p, clamp_id);
> -	struct uclamp_se uc_max = uclamp_default[clamp_id];
> +	struct uclamp_se uc_req, uc_max;
> +
> +	/* Sync up any change to sysctl_sched_uclamp_util_min_rt_default. */

same here: the comment is stating the obvious.

Maybe even just by using a different function name we better document
the code, e.g. uclamp_rt_restrict(p, clamp_id);

This will implicitly convey the purpose: RT tasks can be somehow
further restricted, i.e. in addition to the TG restriction following.


> +	uclamp_sync_util_min_rt_default(p, clamp_id);
> +
> +	uc_req = uclamp_tg_restrict(p, clamp_id);
> +	uc_max = uclamp_default[clamp_id];
>  
>  	/* System default restrictions always apply */
>  	if (unlikely(uc_req.value > uc_max.value))

[...]