[Xen-devel] Re: Regression in 3.1 causes Xen to use wrong idle routine

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 26.10.2011 15:30, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 26, 2011 at 12:24:17PM +0200, Stefan Bader wrote:
>> The following commit changes calls to pm_idle into first trying
>> cpuidle_call_idle() and if that returns non-zero to fall back to
>> call pm_idle().
>>
>> commit a0bfa1373859e9d11dc92561a8667588803e42d8
>> Author: Len Brown <len.brown@xxxxxxxxx>
>> Date:   Fri Apr 1 19:34:59 2011 -0400
>>
>>     cpuidle: stop depending on pm_idle
>>
>> However cpuidle_call_idle() will return -ENODEV if it is supposed to be 
>> disabled
>> by cpuidle.off. Which then causes pm_idle() to be called.
>>
>> This has some bad interaction with the following change that tries to
>> make use of disabling cpuidle in Xen to fall back to hlt.
>>
>> commit d91ee5863b71e8c90eaf6035bff3078a85e2e7b5
>> Author: Len Brown <len.brown@xxxxxxxxx>
>> Date:   Fri Apr 1 18:28:35 2011 -0400
>>
>>     cpuidle: replace xen access to x86 pm_idle and default_idle
>>
>> The problem I see is that select_idle_routine() is called from
>> arch/x86/kernel/cpu/common.c and since Xen setup does not set pm_idle
>> anymore, it can cause mwait_idle or amd_e400_idle functions to be selected.
> 
> Right, b/c that is what d91ee5863b71e8c90eaf6035bff3078a85e2e7b5 was suppose
> to do - " xen scribble on pm_idle and access default_idle,
>     have it simply disable_cpuidle() so acpi_idle will not load and
>     architecture default HLT will be used."
> 
> But it seems that select_idle_routine() was not thought off.
> 
>> In testing it seem amd_e400_idle in PVM domU at least does not immediately 
>> cause
>> problems, but mwait_idle just causes crashes. From the reports I have
>> this may be related to older hypervisors (3.1 and older) not clearing the 
>> mwait
>> capability. But overall there seems something wrong in the interaction.
>>
>> I am not really sure whether the logic of calling pm_idle() on all errors 
>> from
>> cpuidle_call_idle() is already flawed or the assumption in the Xen patch 
>> about
>> being able to prevent the wrong idle function by turning cpuidle off is 
>> incorrect.
>> One quick fix could be to add some Xen case into select_idle_routine() which
>> picks default_idle...
> 
> What about using the cpuidle_disabled() functionality and adhere to that?
> As so:
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index e7e3b01..1f7f8c8 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -14,6 +14,7 @@
>  #include <linux/utsname.h>
>  #include <trace/events/power.h>
>  #include <linux/hw_breakpoint.h>
> +#include <linux/cpuidle.h>
>  #include <asm/cpu.h>
>  #include <asm/system.h>
>  #include <asm/apic.h>
> @@ -587,6 +588,10 @@ void __cpuinit select_idle_routine(const struct 
> cpuinfo_x86 *c)
>       if (pm_idle)
>               return;
>  
> +     if (cpuidle_disabled()) {
> +             pm_idle = default_idle;
> +             return;
> +     }
>       if (cpu_has(c, X86_FEATURE_MWAIT) && mwait_usable(c)) {
>               /*
>                * One CPU supports mwait => All CPUs supports mwait
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index b51629e..123fe9e 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -122,6 +122,7 @@ struct cpuidle_driver {
>  };
>  
>  #ifdef CONFIG_CPU_IDLE
> +extern int cpuidle_disabled(void);
>  extern void disable_cpuidle(void);
>  extern int cpuidle_idle_call(void);
>  
> @@ -137,6 +138,7 @@ extern int cpuidle_enable_device(struct cpuidle_device 
> *dev);
>  extern void cpuidle_disable_device(struct cpuidle_device *dev);
>  
>  #else
> +static inline int cpuidle_disabled(void) { return 1; }
>  static inline void disable_cpuidle(void) { }
>  static inline int cpuidle_idle_call(void) { return -ENODEV; }
>  
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>From reading over it, this should work. Though I would be interested to hear
from the linux-acpi folks. Also to double check that calling pm_idle when
cpuidle.off was specified really is what is intended.

-Stefan

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