linux-kernel - Re: [PATCH v3 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

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Date:   Thu, 30 Apr 2020 20:21:31 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        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>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] sched/uclamp: Add a new sysctl to control RT
 default boost value

On 29/04/2020 14:30, Qais Yousef wrote:
> Hi Pavan
> 
> On 04/29/20 17:02, Pavan Kondeti wrote:
>> Hi Qais,
>>
>> On Tue, Apr 28, 2020 at 05:41:33PM +0100, Qais Yousef wrote:

[...]

>>> @@ -907,8 +935,15 @@ 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 value.
>>> +	 */
>>> +	uclamp_sync_util_min_rt_default(p);
>>> +
>>> +	uc_req = uclamp_tg_restrict(p, clamp_id);
>>> +	uc_max = uclamp_default[clamp_id];
>>
>> We are calling uclamp_sync_util_min_rt_default() unnecessarily for
>> clamp_id == UCLAMP_MAX case. Would it be better to have a separate
> 
> It was actually intentional to make sure we update the value ASAP. I didn't
> think it's a lot of overhead. I can further protect with a check to verify
> whether the value has changed if it seems heavy handed.

Users of uclamp_eff_value()->uclamp_eff_get() ((like
rt_task_fits_capacity())) always call both ids.

So calling uclamp_sync_util_min_rt_default() only for UCLAMP_MIN would
make sense. It's overhead in the fast path for rt tasks.

Since changes to sched_util_clamp_min_rt_default will be fairly rare,
you might even want to consider only doing the uclamp_se_set(...,
min_rt_default, ...) in case

  uc_se->value != sysctl_sched_uclamp_util_min_rt_default

[...]