linux-kernel - Re: [PATCHv2 3/5] cpuidle: add support for states that affect multiple cpus

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Date:	Thu, 15 Mar 2012 17:04:43 -0700
From:	Kevin Hilman <khilman@...com>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	linux-pm@...ts.linux-foundation.org,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2 3/5] cpuidle: add support for states that affect multiple cpus

Colin Cross <ccross@...roid.com> writes:

> +/**
> + * cpuidle_coupled_cpu_set_alive - adjust alive_count during hotplug transitions
> + * @cpu: target cpu number
> + * @alive: whether the target cpu is going up or down
> + *
> + * Run on the cpu that is bringing up the target cpu, before the target cpu
> + * has been booted, or after the target cpu is completely dead.
> + */
> +static void cpuidle_coupled_cpu_set_alive(int cpu, bool alive)
> +{
> +	struct cpuidle_device *dev;
> +	struct cpuidle_coupled *coupled;
> +
> +	mutex_lock(&cpuidle_lock);
> +
> +	dev = per_cpu(cpuidle_devices, cpu);
> +	if (!dev->coupled)
> +		goto out;
> +
> +	coupled = dev->coupled;
> +
> +	/*
> +	 * waiting_count must be at least 1 less than alive_count, because
> +	 * this cpu is not waiting.  Spin until all cpus have noticed this cpu
> +	 * is not idle and exited the ready loop before changing alive_count.
> +	 */
> +	while (atomic_read(&coupled->ready_count))
> +		cpu_relax();
> +
> +	smp_mb__before_atomic_inc();
> +	atomic_inc(&coupled->alive_count);

This doesn't look quite right.  alive_count is incrmented whether the
CPU is going up or down?

Maybe I misunderstood something, but I don't see anywhere where
alive_count is decrmemented after a CPU is removed.

Kevin

> +	smp_mb__after_atomic_inc();
> +
> +	if (alive)
> +		coupled->requested_state[dev->cpu] = CPUIDLE_COUPLED_NOT_IDLE;
> +	else
> +		coupled->requested_state[dev->cpu] = CPUIDLE_COUPLED_DEAD;
> +
> +out:
> +	mutex_unlock(&cpuidle_lock);
> +}
> +
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