Re: [PATCH powerpc] fix a lockdep complaint in start_secondary

[Deepthi Dharwar <deepthi@linux.vnet.ibm.com>]

On 05/18/2012 08:24 AM, Li Zhong wrote:

> On Thu, 2012-05-17 at 15:52 +0530, Deepthi Dharwar wrote:
>> On 05/17/2012 09:58 AM, Benjamin Herrenschmidt wrote:
>>
>>> On Thu, 2012-05-17 at 12:01 +0800, Li Zhong wrote:
>>>> This patch tries to fix following lockdep complaints:
>>>
>>>  .../...
>>>
>>>> pseries_notify_cpu_idle_add_cpu() actually does
>>>> cpuidle_disable_device(), and then cpuidle_enable_device(), which
>>>> releases and allocates the resources respectively. ( Also, all the data
>>>> are cleared and reinitialized after this cycle). The problem here is:
>>>> something like kzalloc(GFP_KERNEL), wait_for_completion() would have
>>>> problems running here where irqs are still disabled. 
>>
>>
>> This is true when the system is booting up.
>>
>>>
>>> So yes, it looks definitely fishy. I don't have time to study cpuidle
>>> today to check whether that's correct or not so I'm CCing Deepthi
>>> Dharwar who did all that cpuidle work for pseries.
>>>
>>> Deepthi, can you check whether that patch is correct ?
>>
>>
>> pseries_notify_cpu_idle_add_cpu() is essential to be called for
>> hotplug event. So by removing this call completely wouldn't
>> support cpus registering under cpuidle on hotplug and default idle is
>> executed on those with do not give much powersavings. 
> 
> Maybe I missed that part.. would you please give some details how
> removing this would prevent powersaving cpuidle being called after
> hotplug? 
> 
> After rereading the codes, I think ppc_md.power_save() is the one you
> mentioned that could give much powersavings? 
> 
> It is registered as pSeries_idle(), which calls cpuidle_idle_call(). 
> It seems to me that it would still be called after hotplug. 
> 
> Or maybe I misunderstood your point? 


If cpuidle_idle_call() fails, in case device is not present, off ,
not initialized and not ready to use, default idle is called.
Coming out of a hotplug event, it is good to cleanly exit out
and reallocate all the resources when needed, rather than using the
stale one to make sure this call succeeds always.

Default idle executed in pSeries_idle() :
		HMT_low();
                HMT_very_low();
This would not have much powersavings.

CPUIDLE subsystem needs to be informed when a hot plug event occurs
and not a good practice to mask this subsystem from this system wide
event.

I agree that putting it in xics setup is not a good thing.
Notifier would be a cleaner way of doing it.
That way, duplication of resources allocated and released at boot
time is not done.
		

> 
>> Ideal way it to
>> have a notifier in pseries backend driver for hotplug notification and
>> then remove this function from here.
>> I am currently working on this patch, will post it out soon.
>>
>>>
>>>> Actually, cpuidle_enable_device() is called for each possible cpu when
>>>> the driver is registered. So I don't think the resources needed to be
>>>> released and allocated each time cpu becomes online. Something like
>>>> cpuidle_reset_device() would be enough to clear and reinitialize the
>>>> data.
>>>>
>>>> However, after some studying of the data to be cleared, I think it's
>>>> also reasonable to keep the previous data. For example: 
>>>>
>>>> /sys/devices/system/cpu/cpu#/cpuidle/state#/usage 
>>>>         the number of times this idle state has been entered
>>>> /sys/devices/system/cpu/cpu#/cpuidle/state#/time
>>>>         the amount of time spent in this idle state
>>>>
>>>> So I think we could just remove the function call doing the
>>>> disable/enable cycle:
>>>>
>>>> Please correct me if I missed anything.
>>
>>
>> If removed, this would not handle cpu hotplug events for cpuidle.
>>
>>
>>>>
>>>> Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>>> Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
>>>> Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>>> ---
>>>>  arch/powerpc/platforms/pseries/smp.c |    1 -
>>>>  1 files changed, 0 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/platforms/pseries/smp.c
>>>> b/arch/powerpc/platforms/pseries/smp.c
>>>> index e16bb8d..71706bc 100644
>>>> --- a/arch/powerpc/platforms/pseries/smp.c
>>>> +++ b/arch/powerpc/platforms/pseries/smp.c
>>>> @@ -147,7 +147,6 @@ static void __devinit smp_xics_setup_cpu(int cpu)
>>>>  	set_cpu_current_state(cpu, CPU_STATE_ONLINE);
>>>>  	set_default_offline_state(cpu);
>>>>  #endif
>>>> -	pseries_notify_cpuidle_add_cpu(cpu);
>>>>  }
>>>>  
>>>>  static int __devinit smp_pSeries_kick_cpu(int nr)
>>>
>>>
>>>
>>
>> Cheers,
>> Deepthi
> 
> 
>