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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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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