[PATCH v5] cpufreq: fix governor start/stop race condition

[PATCH v5] cpufreq: fix governor start/stop race condition

[Xiaoguang Chen]

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.
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.

2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
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
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.

Signed-off-by: Xiaoguang Chen <chenxg@marvell.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 */
 
 	struct work_struct	update; /* if update_policy() needs to be
 					 * called, but you're in IRQ context */
-- 
1.8.0

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Viresh Kumar]

On 13 June 2013 14:31, Xiaoguang Chen <chenxg@marvell.com> 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.
> 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.
>
> 2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
> 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
> 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.
>
> Signed-off-by: Xiaoguang Chen <chenxg@marvell.com>
> ---
>  drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++++++++
>  include/linux/cpufreq.h   |  1 +
>  2 files changed, 25 insertions(+)

I forgot to tell you earlier but please update changelog everytime you
send a new version.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Xiaoguang Chen]

2013/6/13 Viresh Kumar <viresh.kumar@linaro.org>:
> On 13 June 2013 14:31, Xiaoguang Chen <chenxg@marvell.com> 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.
>> 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.
>>
>> 2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
>> 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
>> 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.
>>
>> Signed-off-by: Xiaoguang Chen <chenxg@marvell.com>
>> ---
>>  drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++++++++
>>  include/linux/cpufreq.h   |  1 +
>>  2 files changed, 25 insertions(+)
>
> I forgot to tell you earlier but please update changelog everytime you
> send a new version.
>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

Thanks
I'll pay attention next time :)

Xiaoguang

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Rafael J. Wysocki]

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.

[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?

> 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?

> 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@marvell.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.

>  
>  	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.

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Xiaoguang Chen]

2013/6/19 Rafael J. Wysocki <rjw@sisk.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@marvell.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.

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Viresh Kumar]

On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
> 2013/6/19 Rafael J. Wysocki <rjw@sisk.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.]

Just wanted to know the reasoning behind it so that I can remind
others about it and then argue :)

>>> 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.

Why? This can be for a single cpu (which would eventually force all
others CPUs sharing policy with it).

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Rafael J. Wysocki]

On Wednesday, June 19, 2013 08:30:11 AM Viresh Kumar wrote:
> On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
> > 2013/6/19 Rafael J. Wysocki <rjw@sisk.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.]
> 
> Just wanted to know the reasoning behind it so that I can remind
> others about it and then argue :)

Well, sentences that don't start from upper-case letters don't look like
separate sentences and when I see "cpus" written like this I tend to read
it as "cups" with a typo.

Generally speaking, don't make changelogs harder to read than they have to be.

They usually are hard enough to read anyway.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Viresh Kumar]

On 20 June 2013 04:19, Rafael J. Wysocki <rjw@sisk.pl> wrote:

> Well, sentences that don't start from upper-case letters don't look like
> separate sentences

s/sequences/sentences for your initial mail.

I misunderstood it as, you want to name cpu{0|1|2} as CPU{1|2|3}..
Obviously sentences must start with capitals.

> and when I see "cpus" written like this I tend to read
> it as "cups" with a typo.

Hmm..

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Viresh Kumar]

On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
> 2013/6/19 Rafael J. Wysocki <rjw@sisk.pl>:

>>> 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?

Please read all the questions carefully. You missed this one.

Actually you should write: Current governor is userspace, now cpu0 hot-unplugs
cpu3, it will **

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Viresh Kumar]

On 19 June 2013 08:43, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
>> 2013/6/19 Rafael J. Wysocki <rjw@sisk.pl>:
>
>>>> 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?
>
> Please read all the questions carefully. You missed this one.
>
> Actually you should write: Current governor is userspace, now cpu0 hot-unplugs
> cpu3, it will **

Ahh I am mistaken, you are actually bringing cpu3 back to the system. Then you
must have mentioned earlier that cpu3 wasn't online.

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Xiaoguang Chen]

2013/6/19 Viresh Kumar <viresh.kumar@linaro.org>:
> On 19 June 2013 08:43, Viresh Kumar <viresh.kumar@linaro.org> wrote:
>> On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
>>> 2013/6/19 Rafael J. Wysocki <rjw@sisk.pl>:
>>
>>>>> 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?

Sorry I missed this question, Let me explain it in detail
Suppose we are in such condtition, current cpufreq goveror is
userspace governor. cpu3 is offline.
and two things happen as above two cases, first thing is application
tries to change current governor to ondemand governor,
second thing is cpu0 tries to make cpu3 online which is off line
before. both of these two cases will try to stop current governor and
start a governor. if above two things interleave, unexpected behavior
will happen.


>>
>> Please read all the questions carefully. You missed this one.
>>
>> Actually you should write: Current governor is userspace, now cpu0 hot-unplugs
>> cpu3, it will **
>
> Ahh I am mistaken, you are actually bringing cpu3 back to the system. Then you
> must have mentioned earlier that cpu3 wasn't online.

Re: [PATCH v5] cpufreq: fix governor start/stop race condition

[Rafael J. Wysocki]

On Wednesday, June 19, 2013 11:26:13 AM Xiaoguang Chen wrote:
> 2013/6/19 Viresh Kumar <viresh.kumar@linaro.org>:
> > On 19 June 2013 08:43, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >> On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@gmail.com> wrote:
> >>> 2013/6/19 Rafael J. Wysocki <rjw@sisk.pl>:
> >>
> >>>>> 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?
> 
> Sorry I missed this question, Let me explain it in detail
> Suppose we are in such condtition, current cpufreq goveror is
> userspace governor. cpu3 is offline.
> and two things happen as above two cases, first thing is application
> tries to change current governor to ondemand governor,
> second thing is cpu0 tries to make cpu3 online which is off line
> before. both of these two cases will try to stop current governor and
> start a governor. if above two things interleave, unexpected behavior
> will happen.

Now it's clear, thanks for the explanation.  [Well, I'll still need to fix up
the changelog.  Sigh.]

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.