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

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Date:	Thu, 15 Mar 2012 17:20:08 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Kevin Hilman <khilman@...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

On Thu, Mar 15, 2012 at 5:04 PM, Kevin Hilman <khilman@...com> wrote:
> 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.

Oops, dropped the atomic_dec when I merged from two separate functions
for up and down to a single function that takes a bool.

>> +     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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