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Date:   Mon, 11 May 2020 18:18:49 +0100
From:   Qais Yousef <qais.yousef@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>
Cc:     Randy Dunlap <rdunlap@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Quentin Perret <qperret@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
 boost value
Message-ID: <20200511171849.55rsnpdbqdgopnxs@e107158-lin.cambridge.arm.com>
References: <20200511154053.7822-1-qais.yousef@arm.com>
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Sorry I forgot to label this as v5 in the subject line.

--
Qais Yousef

On 05/11/20 16:40, Qais Yousef wrote:
> RT tasks by default run at the highest capacity/performance level. When
> uclamp is selected this default behavior is retained by enforcing the
> requested uclamp.min (p->uclamp_req[UCLAMP_MIN]) of the RT tasks to be
> uclamp_none(UCLAMP_MAX), which is SCHED_CAPACITY_SCALE; the maximum
> value.
> 
> This is also referred to as 'the default boost value of RT tasks'.
> 
> See commit 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks").
> 
> On battery powered devices, it is desired to control this default
> (currently hardcoded) behavior at runtime to reduce energy consumed by
> RT tasks.
> 
> For example, a mobile device manufacturer where big.LITTLE architecture
> is dominant, the performance of the little cores varies across SoCs, and
> on high end ones the big cores could be too power hungry.
> 
> Given the diversity of SoCs, the new knob allows manufactures to tune
> the best performance/power for RT tasks for the particular hardware they
> run on.
> 
> They could opt to further tune the value when the user selects
> a different power saving mode or when the device is actively charging.
> 
> The runtime aspect of it further helps in creating a single kernel image
> that can be run on multiple devices that require different tuning.
> 
> Keep in mind that a lot of RT tasks in the system are created by the
> kernel. On Android for instance I can see over 50 RT tasks, only
> a handful of which created by the Android framework.
> 
> To control the default behavior globally by system admins and device
> integrators, introduce the new sysctl_sched_uclamp_util_min_rt_default
> to change the default boost value of the RT tasks.
> 
> I anticipate this to be mostly in the form of modifying the init script
> of a particular device.
> 
> Whenever the new default changes, it'd be applied lazily on the next
> opportunity the scheduler needs to calculate the effective uclamp.min
> value for the task, assuming that it still uses the system default value
> and not a user applied one.
> 
> Tested on Juno-r2 in combination with the RT capacity awareness [1].
> By default an RT task will go to the highest capacity CPU and run at the
> maximum frequency, which is particularly energy inefficient on high end
> mobile devices because the biggest core[s] are 'huge' and power hungry.
> 
> With this patch the RT task can be controlled to run anywhere by
> default, and doesn't cause the frequency to be maximum all the time.
> Yet any task that really needs to be boosted can easily escape this
> default behavior by modifying its requested uclamp.min value
> (p->uclamp_req[UCLAMP_MIN]) via sched_setattr() syscall.
> 
> [1] 804d402fb6f6: ("sched/rt: Make RT capacity-aware")
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> CC: Jonathan Corbet <corbet@lwn.net>
> CC: Juri Lelli <juri.lelli@redhat.com>
> CC: Vincent Guittot <vincent.guittot@linaro.org>
> CC: Dietmar Eggemann <dietmar.eggemann@arm.com>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ben Segall <bsegall@google.com>
> CC: Mel Gorman <mgorman@suse.de>
> CC: Luis Chamberlain <mcgrof@kernel.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: Iurii Zaikin <yzaikin@google.com>
> CC: Quentin Perret <qperret@google.com>
> CC: Valentin Schneider <valentin.schneider@arm.com>
> CC: Patrick Bellasi <patrick.bellasi@matbug.net>
> CC: Pavan Kondeti <pkondeti@codeaurora.org>
> CC: linux-doc@vger.kernel.org
> CC: linux-kernel@vger.kernel.org
> CC: linux-fsdevel@vger.kernel.org
> ---
> 
> Changes in v5 (all from Patrick):
> 	* Remove use of likely/unlikely
> 	* Use short hand variable for sysctl_sched_uclamp_util_min_rt_default
> 	* Combine if conditions that checks for errors when setting
> 	  sysctl_sched_uclamp_util_min_rt_default
> 	* Fit a comment in a single line
> 
> v4 discussion:
> 
> https://lore.kernel.org/lkml/20200501114927.15248-1-qais.yousef@arm.com/
> 
>  include/linux/sched/sysctl.h |  1 +
>  kernel/sched/core.c          | 63 +++++++++++++++++++++++++++++++-----
>  kernel/sysctl.c              |  7 ++++
>  3 files changed, 63 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> index d4f6215ee03f..e62cef019094 100644
> --- a/include/linux/sched/sysctl.h
> +++ b/include/linux/sched/sysctl.h
> @@ -59,6 +59,7 @@ extern int sysctl_sched_rt_runtime;
>  #ifdef CONFIG_UCLAMP_TASK
>  extern unsigned int sysctl_sched_uclamp_util_min;
>  extern unsigned int sysctl_sched_uclamp_util_max;
> +extern unsigned int sysctl_sched_uclamp_util_min_rt_default;
>  #endif
>  
>  #ifdef CONFIG_CFS_BANDWIDTH
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9a2fbf98fd6f..ea1e11db78bb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -790,6 +790,26 @@ unsigned int sysctl_sched_uclamp_util_min = SCHED_CAPACITY_SCALE;
>  /* Max allowed maximum utilization */
>  unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
>  
> +/*
> + * By default RT tasks run at the maximum performance point/capacity of the
> + * system. Uclamp enforces this by always setting UCLAMP_MIN of RT tasks to
> + * SCHED_CAPACITY_SCALE.
> + *
> + * This knob allows admins to change the default behavior when uclamp is being
> + * used. In battery powered devices, particularly, running at the maximum
> + * capacity and frequency will increase energy consumption and shorten the
> + * battery life.
> + *
> + * This knob only affects RT tasks that their uclamp_se->user_defined == false.
> + *
> + * This knob will not override the system default sched_util_clamp_min defined
> + * above.
> + *
> + * Any modification is applied lazily on the next attempt to calculate the
> + * effective value of the task.
> + */
> +unsigned int sysctl_sched_uclamp_util_min_rt_default = SCHED_CAPACITY_SCALE;
> +
>  /* All clamps are required to be less or equal than these values */
>  static struct uclamp_se uclamp_default[UCLAMP_CNT];
>  
> @@ -872,6 +892,28 @@ unsigned int uclamp_rq_max_value(struct rq *rq, enum uclamp_id clamp_id,
>  	return uclamp_idle_value(rq, clamp_id, clamp_value);
>  }
>  
> +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];
> +
> +	/*
> +	 * 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;
> +
> +	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. */
> +	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))
> @@ -1114,12 +1161,13 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  				loff_t *ppos)
>  {
>  	bool update_root_tg = false;
> -	int old_min, old_max;
> +	int old_min, old_max, old_min_rt;
>  	int result;
>  
>  	mutex_lock(&uclamp_mutex);
>  	old_min = sysctl_sched_uclamp_util_min;
>  	old_max = sysctl_sched_uclamp_util_max;
> +	old_min_rt = sysctl_sched_uclamp_util_min_rt_default;
>  
>  	result = proc_dointvec(table, write, buffer, lenp, ppos);
>  	if (result)
> @@ -1128,7 +1176,9 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  		goto done;
>  
>  	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max ||
> -	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE) {
> +	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE		||
> +	    sysctl_sched_uclamp_util_min_rt_default > SCHED_CAPACITY_SCALE) {
> +
>  		result = -EINVAL;
>  		goto undo;
>  	}
> @@ -1158,6 +1208,7 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  undo:
>  	sysctl_sched_uclamp_util_min = old_min;
>  	sysctl_sched_uclamp_util_max = old_max;
> +	sysctl_sched_uclamp_util_min_rt_default = old_min_rt;
>  done:
>  	mutex_unlock(&uclamp_mutex);
>  
> @@ -1200,10 +1251,6 @@ static void __setscheduler_uclamp(struct task_struct *p,
>  		if (uc_se->user_defined)
>  			continue;
>  
> -		/* By default, RT tasks always get 100% boost */
> -		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			clamp_value = uclamp_none(UCLAMP_MAX);
> -
>  		uclamp_se_set(uc_se, clamp_value, false);
>  	}
>  
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a176d8727a3..64117363c502 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -453,6 +453,13 @@ static struct ctl_table kern_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= sysctl_sched_uclamp_handler,
>  	},
> +	{
> +		.procname	= "sched_util_clamp_min_rt_default",
> +		.data		= &sysctl_sched_uclamp_util_min_rt_default,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= sysctl_sched_uclamp_handler,
> +	},
>  #endif
>  #ifdef CONFIG_SCHED_AUTOGROUP
>  	{
> -- 
> 2.17.1
>