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Date: Wed, 19 Jun 2013 09:20:38 +0800
From: Xiaoguang Chen <chenxg.marvell@...il.com>
To: "Rafael J. Wysocki" <rjw@...k.pl>
Cc: Xiaoguang Chen <chenxg@...vell.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
njiang1@...vell.com, zjwu@...vell.com, ylmao@...vell.com
Subject: Re: [PATCH v5] cpufreq: fix governor start/stop race condition
2013/6/19 Rafael J. Wysocki <rjw@...k.pl>:
> On Thursday, June 13, 2013 05:01:58 PM Xiaoguang Chen wrote:
>> cpufreq governor stop and start should be kept in sequence.
>> If not, there will be unexpected behavior, for example:
>>
>> we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
>
> Please spell cpus as "CPUs". And please start sequences from capitals.
Ok, thanks for the remind
>
> [Yes, it *really* is a problem.]
>
>> the normal sequence is as below:
>>
>> 1) Current governor is userspace, one application tries to set
>> governor to ondemand. it will call __cpufreq_set_policy in which it
>> will stop userspace governor and then start ondemand governor.
>
> Do I think correctly that this is for all CPUs?
>From current code design, it is for all CPUs.
>
>> 2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
>
> Can you please tell me what the above is supposed to mean? Is it supposed to
> mean "the online of cpu3 is being run on cpu0" or something different? If
> something different, then what?
>
>> call cpufreq_add_policy_cpu. on which it first stops userspace
>> governor, and then starts userspace governor.
>>
>> Now if the sequence of above two cases interleaves, it becames
>> below sequence:
>>
>> 1) application stops userspace governor
>> 2) hotplug stops userspace governor
>
> The problem is already here, right? The governor shouldn't be stopped twice?
Yes, we should make sure governor is started before it is stopped.
>
>> 3) application starts ondemand governor
>> 4) hotplug starts a governor
>>
>> in step 4, hotplug is supposed to start userspace governor, but now
>> the governor has been changed by application to ondemand, so hotplug
>> starts ondemand governor again !!!!
>>
>> The solution is: do not allow stop one policy's governor multi-times
>> Governor stop should only do once for one policy, after it is stopped,
>> no other governor stop should be executed. also add one mutext to
>> protect __cpufreq_governor so governor operation can be kept in sequence.
>
> One more request. ->
>
>> Signed-off-by: Xiaoguang Chen <chenxg@...vell.com>
>> ---
>> drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++++++++
>> include/linux/cpufreq.h | 1 +
>> 2 files changed, 25 insertions(+)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 2d53f47..b51473e 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -46,6 +46,7 @@ static DEFINE_PER_CPU(struct cpufreq_policy *, cpufreq_cpu_data);
>> static DEFINE_PER_CPU(char[CPUFREQ_NAME_LEN], cpufreq_cpu_governor);
>> #endif
>> static DEFINE_RWLOCK(cpufreq_driver_lock);
>> +static DEFINE_MUTEX(cpufreq_governor_lock);
>>
>> /*
>> * cpu_policy_rwsem is a per CPU reader-writer semaphore designed to cure
>> @@ -1562,6 +1563,21 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>>
>> pr_debug("__cpufreq_governor for CPU %u, event %u\n",
>> policy->cpu, event);
>> +
>> + mutex_lock(&cpufreq_governor_lock);
>> + if ((!policy->governor_enabled && (event == CPUFREQ_GOV_STOP)) ||
>> + (policy->governor_enabled && (event == CPUFREQ_GOV_START))) {
>> + mutex_unlock(&cpufreq_governor_lock);
>> + return -EBUSY;
>> + }
>> +
>> + if (event == CPUFREQ_GOV_STOP)
>> + policy->governor_enabled = 0;
>> + else if (event == CPUFREQ_GOV_START)
>> + policy->governor_enabled = 1;
>> +
>> + mutex_unlock(&cpufreq_governor_lock);
>> +
>> ret = policy->governor->governor(policy, event);
>>
>> if (!ret) {
>> @@ -1569,6 +1585,14 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>> policy->governor->initialized++;
>> else if (event == CPUFREQ_GOV_POLICY_EXIT)
>> policy->governor->initialized--;
>> + } else {
>> + /* Restore original values */
>> + mutex_lock(&cpufreq_governor_lock);
>> + if (event == CPUFREQ_GOV_STOP)
>> + policy->governor_enabled = 1;
>> + else if (event == CPUFREQ_GOV_START)
>> + policy->governor_enabled = 0;
>> + mutex_unlock(&cpufreq_governor_lock);
>> }
>>
>> /* we keep one module reference alive for
>> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
>> index 037d36a..c12db73 100644
>> --- a/include/linux/cpufreq.h
>> +++ b/include/linux/cpufreq.h
>> @@ -107,6 +107,7 @@ struct cpufreq_policy {
>> unsigned int policy; /* see above */
>> struct cpufreq_governor *governor; /* see below */
>> void *governor_data;
>> + int governor_enabled; /* governor start/stop flag */
>
> -> Please use bool here and true/false instead of 1/0 above.
>
Ok, I'll change it to bool.
>>
>> struct work_struct update; /* if update_policy() needs to be
>> * called, but you're in IRQ context */
>
> Thanks,
> Rafael
>
>
> --
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
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