linux-kernel - Re: [PATCH v4 7/7] cpufreq: schedutil: New governor based on scheduler utilization data

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Message-ID: <20160316175211.GF6344@twins.programming.kicks-ass.net>
Date:	Wed, 16 Mar 2016 18:52:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 7/7] cpufreq: schedutil: New governor based on
 scheduler utilization data

On Wed, Mar 16, 2016 at 03:59:18PM +0100, Rafael J. Wysocki wrote:
> +static void sugov_work(struct work_struct *work)
> +{
> +	struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> +
> +	mutex_lock(&sg_policy->work_lock);
> +	__cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> +				CPUFREQ_RELATION_L);
> +	mutex_unlock(&sg_policy->work_lock);
> +

Be aware that the below store can creep up and become visible before the
unlock. AFAICT that doesn't really matter, but still.

> +	sg_policy->work_in_progress = false;
> +}
> +
> +static void sugov_irq_work(struct irq_work *irq_work)
> +{
> +	struct sugov_policy *sg_policy;
> +
> +	sg_policy = container_of(irq_work, struct sugov_policy, irq_work);
> +	schedule_work(&sg_policy->work);
> +}

If you care what cpu the work runs on, you should schedule_work_on(),
regular schedule_work() can end up on any random cpu (although typically
it does not).

In particular schedule_work() -> queue_work() -> queue_work_on(.cpu =
WORK_CPU_UNBOUND) -> __queue_work() if (req_cpu == UNBOUND) cpu =
wq_select_unbound_cpu(), which has a Round-Robin 'feature' to detect
just such dependencies.