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Date:   Mon, 21 Jan 2019 16:33:08 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.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 v6 05/16] sched/core: uclamp: Update CPU's refcount on
 clamp changes
Message-ID: <20190121153308.GL27931@hirez.programming.kicks-ass.net>
References: <20190115101513.2822-1-patrick.bellasi@arm.com>
 <20190115101513.2822-6-patrick.bellasi@arm.com>
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On Tue, Jan 15, 2019 at 10:15:02AM +0000, Patrick Bellasi wrote:

> +static inline void
> +uclamp_task_update_active(struct task_struct *p, unsigned int clamp_id)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +
> +	/*
> +	 * Lock the task and the CPU where the task is (or was) queued.
> +	 *
> +	 * We might lock the (previous) rq of a !RUNNABLE task, but that's the
> +	 * price to pay to safely serialize util_{min,max} updates with
> +	 * enqueues, dequeues and migration operations.
> +	 * This is the same locking schema used by __set_cpus_allowed_ptr().
> +	 */
> +	rq = task_rq_lock(p, &rf);
> +
> +	/*
> +	 * Setting the clamp bucket is serialized by task_rq_lock().
> +	 * If the task is not yet RUNNABLE and its task_struct is not
> +	 * affecting a valid clamp bucket, the next time it's enqueued,
> +	 * it will already see the updated clamp bucket value.
> +	 */
> +	if (!p->uclamp[clamp_id].active)
> +		goto done;
> +
> +	uclamp_cpu_dec_id(p, rq, clamp_id);
> +	uclamp_cpu_inc_id(p, rq, clamp_id);
> +
> +done:
> +	task_rq_unlock(rq, p, &rf);
> +}

> @@ -1008,11 +1043,11 @@ static int __setscheduler_uclamp(struct task_struct *p,
>  
>  	mutex_lock(&uclamp_mutex);
>  	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> -		uclamp_bucket_inc(&p->uclamp[UCLAMP_MIN],
> +		uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MIN],
>  				  UCLAMP_MIN, lower_bound);
>  	}
>  	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> -		uclamp_bucket_inc(&p->uclamp[UCLAMP_MAX],
> +		uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MAX],
>  				  UCLAMP_MAX, upper_bound);
>  	}
>  	mutex_unlock(&uclamp_mutex);


But.... __sched_setscheduler() actually does the whole dequeue + enqueue
thing already ?!? See where it does __setscheduler().