Re: [PATCH v4 2/2] CPU hotplug, Freezer: Synchronize CPU hotplugand Freezer

[Srivatsa S. Bhat]

On 10/28/2011 05:27 PM, Rafael J. Wysocki wrote:
> On Friday, October 28, 2011, Srivatsa S. Bhat wrote:
>> On 10/28/2011 01:43 AM, Rafael J. Wysocki wrote:
>>> Hi,
>>>
>>> On Thursday, October 27, 2011, Srivatsa S. Bhat wrote:
>>>> Prevent CPU hotplug and the freezer from racing with each other, to ensure
>>>> that during the *entire duration* for which the callbacks for CPU hotplug
>>>> notifications such as CPU_ONLINE[_FROZEN], CPU_DEAD[_FROZEN] etc are being
>>>> executed, the state of the system (with respect to the tasks being frozen
>>>> or not) remains constant.
>>>>
>>>> This patches hooks the CPU hotplug infrastructure onto the freezer
>>>> notifications (PM_FREEZE_PREPARE and PM_POST_THAW) and thus synchronizes
>>>> with the freezer.
>>>>
>>>> Specifically,
>>>>
>>>> * Upon the PM_FREEZE_PREPARE notification, the CPU hotplug callback disables
>>>>   future (regular) CPU hotplugging and also ensures that any currently running
>>>>   CPU hotplug operation is completed before allowing the freezer to continue
>>>>   any further.
>>>>
>>>> * Upon the PM_POST_THAW notification, the CPU hotplug callback re-enables
>>>>   regular CPU hotplug.
>>>>
>>>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>>>> ---
>>>>
>>>>  kernel/cpu.c |   76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  1 files changed, 76 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>>>> index 12b7458..61985ce 100644
>>>> --- a/kernel/cpu.c
>>>> +++ b/kernel/cpu.c
>>>> @@ -15,6 +15,7 @@
>>>>  #include <linux/stop_machine.h>
>>>>  #include <linux/mutex.h>
>>>>  #include <linux/gfp.h>
>>>> +#include <linux/suspend.h>
>>>>  
>>>>  #ifdef CONFIG_SMP
>>>>  /* Serializes the updates to cpu_online_mask, cpu_present_mask */
>>>> @@ -478,6 +479,81 @@ static int alloc_frozen_cpus(void)
>>>>  core_initcall(alloc_frozen_cpus);
>>>>  #endif /* CONFIG_PM_SLEEP_SMP */
>>>>  
>>>> +
>>>> +#ifdef CONFIG_FREEZER
>>>> +
>>>> +/*
>>>> + * Avoid CPU hotplug racing with the freezer subsystem, by disabling CPU
>>>> + * hotplug when tasks are about to be frozen.
>>>> + *
>>>> + * Also, don't allow the freezer subsystem to continue until any currently
>>>> + * running CPU hotplug operation gets completed.
>>>> + * To modify the 'cpu_hotplug_disabled' flag, we need to acquire the
>>>> + * 'cpu_add_remove_lock'. And this same lock is also taken by the regular
>>>> + * CPU hotplug path and released only after it is complete. Thus, we
>>>> + * (and hence the freezer) will block here until any currently running CPU
>>>> + * hotplug operation is completed.
>>>> + */
>>>> +static void cpu_hotplug_freezer_block_begin(void)
>>>> +{
>>>> +	cpu_maps_update_begin();
>>>> +	cpu_hotplug_disabled = 1;
>>>> +	cpu_maps_update_done();
>>>> +}
>>>> +
>>>> +
>>>> +/*
>>>> + * When thawing of tasks is complete, re-enable CPU hotplug (which had been
>>>> + * disabled while beginning to freeze tasks).
>>>> + */
>>>> +static void cpu_hotplug_freezer_block_done(void)
>>>> +{
>>>> +	cpu_maps_update_begin();
>>>> +	cpu_hotplug_disabled = 0;
>>>> +	cpu_maps_update_done();
>>>> +}
>>>> +
>>>
>>> I wonder if the new PM notifier events are really necessary?
>>>
>>> Why don't you just call cpu_hotplug_freezer_block_begin() (perhaps
>>> with a better name?) directly from freeze_processes()?  And analogously
>>> for cpu_hotplug_freezer_block_done() and thaw_processes()?
>>>
>>
>> Yes, we can definitely do that. 
>>
>> But the reason why I chose to introduce new notifiers was to make this
>> more extensible (because we know that at least 2 subsystems would benefit
>> from mutually excluding themselves from the freezer, namely CPU hotplug
>> and x86 microcode).
>> http://thread.gmane.org/gmane.linux.kernel/1198291/focus=1200591
>>
>> But now that I think of it, hooking onto the freezer notifiers wouldn't
>> solve the microcode cases since usermodehelper_disable() is called
>> _before_ freezing tasks... :(
>>
>> So we should probably call the functions directly like you suggested.. 
>>
>> But I really didn't want to clutter the freezer call path because of problems
>> elsewhere. So I felt freezer notifiers would be a cleaner way of dealing with
>> such things. Also, since freezer is a generic subsystem that could be used
>> for purposes other than S3/S4 as well (I have heard of attempts to use freezer
>> during tracing), wouldn't it be better to introduce new notifiers to
>> announce the begin and end of freezer activity to interested subsystems?
>> (and then use them to solve the CPU hotplug issue like this patch does...)
>>
>> Please let me know your suggestions.
> 
> The freeze_processes() and thaw_processes() functions are only used for
> system suspend and hibernation, as far as I can tell, and I don't think there
> will be any other users in predictable future.
> 
> Also, adding the calls directly to those functions will show exactly what
> the dependecies are, while doing that through a notifier kind of obfuscates
> things.  So, please make direct calls from there.
> 

Ok, thank you for the clarification. I'll post the next version of the patch
with direct function calls.

-- 
Regards,
Srivatsa S. Bhat  <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
Linux Technology Center,
IBM India Systems and Technology Lab
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