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From: Patrick Bellasi <patrick.bellasi@arm.com>
To: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paul Turner <pjt@google.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX
Date: Thu, 16 Aug 2018 18:20:17 +0100
Message-ID: <20180816172016.GG2960@e110439-lin> (raw)
In-Reply-To: <20180806163946.28380-8-patrick.bellasi@arm.com>

On 06-Aug 17:39, Patrick Bellasi wrote:
> When a util_max clamped task sleeps, its clamp constraints are removed
> from the CPU. However, the blocked utilization on that CPU can still be
> higher than the max clamp value enforced while that task was running.
> This max clamp removal when a CPU is going to be idle could thus allow
> unwanted CPU frequency increases, right while the task is not running.
> 
> This can happen, for example, where there is another (smaller) task
> running on a different CPU of the same frequency domain.
> In this case, when we aggregate the utilization of all the CPUs in a
> shared frequency domain, schedutil can still see the full non clamped
> blocked utilization of all the CPUs and thus eventually increase the
> frequency.
> 
> Let's fix this by using:
> 
>    uclamp_cpu_put_id(UCLAMP_MAX)
>       uclamp_cpu_update(last_clamp_value)
> 
> to detect when a CPU has no more RUNNABLE clamped tasks and to flag this
> condition. Thus, while a CPU is idle, we can still enforce the last used
> clamp value for it.
> 
> To the contrary, we do not track any UCLAMP_MIN since, while a CPU is
> idle, we don't want to enforce any minimum frequency
> Indeed, we rely just on blocked load decay to smoothly reduce the
> frequency.

[...]

> @@ -906,7 +906,8 @@ uclamp_group_find(int clamp_id, unsigned int clamp_value)
>   * For the specified clamp index, this method computes the new CPU utilization
>   * clamp to use until the next change on the set of RUNNABLE tasks on that CPU.
>   */
> -static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
> +static inline void uclamp_cpu_update(struct rq *rq, int clamp_id,
> +				     unsigned int last_clamp_value)
>  {
>  	struct uclamp_group *uc_grp = &rq->uclamp.group[clamp_id][0];
>  	int max_value = UCLAMP_NOT_VALID;
> @@ -924,6 +925,19 @@ static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
>  		if (max_value >= SCHED_CAPACITY_SCALE)
>  			break;
>  	}
> +
> +	/*
> +	 * Just for the UCLAMP_MAX value, in case there are no RUNNABLE
> +	 * task, we keep the CPU clamped to the last task's clamp value.
> +	 * This avoids frequency spikes to MAX when one CPU, with an high
> +	 * blocked utilization, sleeps and another CPU, in the same frequency
> +	 * domain, do not see anymore the clamp on the first CPU.
> +	 */
> +	if (clamp_id == UCLAMP_MAX && max_value == UCLAMP_NOT_VALID) {
> +		rq->uclamp.flags |= UCLAMP_FLAG_IDLE;
> +		max_value = last_clamp_value;
> +	}
> +
>  	rq->uclamp.value[clamp_id] = max_value;
>  }
>  
> @@ -953,13 +967,26 @@ static inline void uclamp_cpu_get_id(struct task_struct *p,
>  	uc_grp = &rq->uclamp.group[clamp_id][0];
>  	uc_grp[group_id].tasks += 1;
>  
> +	/* Force clamp update on idle exit */
> +	uc_cpu = &rq->uclamp;
> +	clamp_value = p->uclamp[clamp_id].value;
> +	if (unlikely(uc_cpu->flags & UCLAMP_FLAG_IDLE)) {
> +		/*
> +		 * This function is called for both UCLAMP_MIN (before) and
> +		 * UCLAMP_MAX (after). Let's reset the flag only the second
> +		 * once we know that UCLAMP_MIN has been already updated.
> +		 */
> +		if (clamp_id == UCLAMP_MAX)
> +			uc_cpu->flags &= ~UCLAMP_FLAG_IDLE;
> +		uc_cpu->value[clamp_id] = clamp_value;
> +		return;
> +	}

Just notice that the code block above is not reached when we enqueue a task
without a valid clamp group, i.e. an un-clamped task, which is the
default for all tasks.

The fix should be as simple as moving this block at the beginning of
uclamp_cpu_update() so that we always un-conditionally release the
clamp holding as soon as we enqueue the first task after a CPU wakeup,
i.e. when the UCLAMP_FLAG_IDLE flag is set.

Will fix this on v4.

-- 
#include <best/regards.h>

Patrick Bellasi

  parent reply index

Thread overview: 82+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-06 16:39 [PATCH v3 00/14] Add utilization clamping support Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 01/14] sched/core: uclamp: extend sched_setattr to support utilization clamping Patrick Bellasi
2018-08-06 16:50   ` Randy Dunlap
2018-08-09  8:39     ` Patrick Bellasi
2018-08-09 15:20       ` Randy Dunlap
2018-08-07  9:59   ` Juri Lelli
2018-08-13 12:14     ` Patrick Bellasi
2018-08-13 12:27       ` Juri Lelli
2018-08-07 12:35   ` Juri Lelli
2018-08-09  9:14     ` Patrick Bellasi
2018-08-09  9:50       ` Juri Lelli
2018-08-09 15:23         ` Patrick Bellasi
2018-08-10  7:50           ` Juri Lelli
2018-08-17 10:34           ` Quentin Perret
2018-08-17 10:57             ` Patrick Bellasi
2018-08-17 11:14               ` Quentin Perret
2018-08-06 16:39 ` [PATCH v3 02/14] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups Patrick Bellasi
2018-08-14 11:25   ` Pavan Kondeti
2018-08-14 15:21     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 03/14] sched/core: uclamp: add CPU's clamp groups accounting Patrick Bellasi
2018-08-14 15:44   ` Dietmar Eggemann
2018-08-14 16:49     ` Patrick Bellasi
2018-08-15  9:37       ` Dietmar Eggemann
2018-08-15 10:54         ` Patrick Bellasi
2018-08-15 10:59           ` Dietmar Eggemann
2018-08-16 13:32             ` Patrick Bellasi
2018-08-16 13:37               ` Quentin Perret
2018-08-16 13:45                 ` Dietmar Eggemann
2018-08-16 14:21                   ` Quentin Perret
2018-08-16 15:00                     ` Dietmar Eggemann
2018-08-17 11:04   ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 04/14] sched/core: uclamp: update CPU's refcount on clamp changes Patrick Bellasi
2018-08-15 15:02   ` Dietmar Eggemann
2018-08-16 13:22     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 05/14] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks Patrick Bellasi
2018-08-08 13:18   ` Vincent Guittot
2018-08-09 15:30     ` Patrick Bellasi
2018-08-15 15:30   ` Dietmar Eggemann
2018-08-16 13:53     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks Patrick Bellasi
2018-08-07 13:26   ` Juri Lelli
2018-08-09 15:34     ` Patrick Bellasi
2018-08-09 16:03       ` Vincent Guittot
2018-08-13 10:12         ` Patrick Bellasi
2018-08-13 10:50           ` Juri Lelli
2018-08-13 12:07           ` Vincent Guittot
2018-08-13 12:09             ` Vincent Guittot
2018-08-13 12:49             ` Patrick Bellasi
2018-08-13 14:06               ` Vincent Guittot
2018-08-13 15:01                 ` Patrick Bellasi
2018-08-16 10:34                   ` Dietmar Eggemann
2018-08-16 13:40                     ` Patrick Bellasi
2018-08-07 13:54   ` Quentin Perret
2018-08-09 15:41     ` Patrick Bellasi
2018-08-09 15:55       ` Quentin Perret
2018-08-13 10:17         ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX Patrick Bellasi
2018-08-16 15:43   ` Dietmar Eggemann
2018-08-16 16:47     ` Patrick Bellasi
2018-08-16 17:10       ` Dietmar Eggemann
2018-08-16 17:27         ` Patrick Bellasi
2018-08-16 17:20   ` Patrick Bellasi [this message]
2018-08-06 16:39 ` [PATCH v3 08/14] sched/core: uclamp: extend cpu's cgroup controller Patrick Bellasi
2018-08-17 12:21   ` Dietmar Eggemann
2018-08-17 14:24     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 09/14] sched/core: uclamp: propagate parent clamps Patrick Bellasi
2018-08-16  9:09   ` Pavan Kondeti
2018-08-16 14:07     ` Patrick Bellasi
2018-08-17 13:43   ` Dietmar Eggemann
2018-08-17 14:45     ` Patrick Bellasi
2018-08-17 15:50       ` Dietmar Eggemann
2018-08-20 10:01         ` Dietmar Eggemann
2018-08-20 12:28           ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 10/14] sched/core: uclamp: map TG's clamp values into CPU's clamp groups Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 11/14] sched/core: uclamp: use TG's clamps to restrict Task's clamps Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 12/14] sched/core: uclamp: add system default clamps Patrick Bellasi
2018-08-16  9:13   ` Pavan Kondeti
2018-08-16 14:37     ` Patrick Bellasi
2018-08-20 10:18   ` Dietmar Eggemann
2018-08-20 12:27     ` Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 13/14] sched/core: uclamp: update CPU's refcount on TG's clamp changes Patrick Bellasi
2018-08-06 16:39 ` [PATCH v3 14/14] sched/core: uclamp: use percentage clamp values Patrick Bellasi

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