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Subject: Re: [PATCH] sched/rt: Add a new sysctl to control uclamp_util_min
To:     Quentin Perret <qperret@google.com>,
        Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        valentin.schneider@arm.com,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        kernel-team@android.com
References: <20191220164838.31619-1-qais.yousef@arm.com>
 <20200107134234.GA158998@google.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <8bb17e84-d43f-615f-d04d-c36bb6ede5e0@arm.com>
Date:   Tue, 7 Jan 2020 20:30:36 +0100
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On 07/01/2020 14:42, Quentin Perret wrote:
> Hi Qais,
> 
> On Friday 20 Dec 2019 at 16:48:38 (+0000), Qais Yousef wrote:

[...]

>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index e591d40fd645..19572dfc175b 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -2147,6 +2147,12 @@ static void pull_rt_task(struct rq *this_rq)
>>   */
>>  static void task_woken_rt(struct rq *rq, struct task_struct *p)
>>  {
>> +	/*
>> +	 * When sysctl_sched_rt_uclamp_util_min value is changed by the user,
>> +	 * we apply any new value on the next wakeup, which is here.
>> +	 */
>> +	uclamp_rt_sync_default_util_min(p);
> 
> The task has already been enqueued and sugov has been called by then I
> think, so this is a bit late. You could do that in uclamp_rq_inc() maybe?

That's probably better.
Just to be sure ...we want this feature (an existing rt task gets its
UCLAMP_MIN value set when the sysctl changes) because there could be rt
tasks running before the sysctl is set?

>> +
>>  	if (!task_running(rq, p) &&
>>  	    !test_tsk_need_resched(rq->curr) &&
>>  	    p->nr_cpus_allowed > 1 &&
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 280a3c735935..337bf17b1a9d 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -2300,6 +2300,8 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
>>  #endif /* CONFIG_CPU_FREQ */
>>  
>>  #ifdef CONFIG_UCLAMP_TASK
>> +void uclamp_rt_sync_default_util_min(struct task_struct *p);
>> +
>>  unsigned int uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id);
>>  
>>  static __always_inline
>> @@ -2330,6 +2332,8 @@ static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
>>  	return uclamp_util_with(rq, util, NULL);
>>  }
>>  #else /* CONFIG_UCLAMP_TASK */
>> +void uclamp_rt_sync_default_util_min(struct task_struct *p) {}

-void uclamp_rt_sync_default_util_min(struct task_struct *p) {}
+static inline void uclamp_rt_sync_default_util_min(struct task_struct
*p) {}

[...]