Subject: Re: [RFC PATCH v2 1/4] cpuidle: (powerpc) Add cpu_idle_wait() to allow switching of idle routines

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 28 Nov 2011 09:48:16 +1100

On Thu, 2011-11-17 at 16:58 +0530, Deepthi Dharwar wrote:
> This patch provides cpu_idle_wait() routine for the powerpc
> platform which is required by the cpuidle subsystem. This
> routine is requied to change the idle handler on SMP systems.
> The equivalent routine for x86 is in arch/x86/kernel/process.c
> but the powerpc implementation is different.
> 
> Signed-off-by: Deepthi Dharwar <[email-masked]>
> Signed-off-by: Trinabh Gupta <[email-masked]>
> Signed-off-by: Arun R Bharadwaj <[email-masked]>
> ---

No, that patch also adds this idle boot override thing (can you pick a
shorter name for boot_option_idle_override btw ?) which seems unrelated
and without any explanation as to what it's supposed to be about.

Additionally, I'm a bit worried (but maybe we already discussed that a
while back, I don't know) but cpu_idle_wait() has "wait" in the name,
which makes me think it might need to actually -wait- for all cpus to
have come out of the function.

Now your implementation doesn't provide that guarantee. It might be
fine, I don't know, but if it is, you'd better document it well in the
comments surrounding the code, because as it is, all you do is shoot an
interrupt which will cause the target CPU to eventually come out of idle
some time in the future.

Cheers,
Ben.

>  arch/powerpc/Kconfig                 |    4 ++++
>  arch/powerpc/include/asm/processor.h |    2 ++
>  arch/powerpc/include/asm/system.h    |    1 +
>  arch/powerpc/kernel/idle.c           |   26 ++++++++++++++++++++++++++
>  4 files changed, 33 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index b177caa..87f8604 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -87,6 +87,10 @@ config ARCH_HAS_ILOG2_U64
>  	bool
>  	default y if 64BIT
>  
> +config ARCH_HAS_CPU_IDLE_WAIT
> +	bool
> +	default y
> +
>  config GENERIC_HWEIGHT
>  	bool
>  	default y
> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index eb11a44..811b7e7 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -382,6 +382,8 @@ static inline unsigned long get_clean_sp(struct pt_regs *regs, int is_32)
>  }
>  #endif
>  
> +enum idle_boot_override {IDLE_NO_OVERRIDE = 0, IDLE_POWERSAVE_OFF};
> +
>  #endif /* __KERNEL__ */
>  #endif /* __ASSEMBLY__ */
>  #endif /* _ASM_POWERPC_PROCESSOR_H */
> diff --git a/arch/powerpc/include/asm/system.h b/arch/powerpc/include/asm/system.h
> index e30a13d..ff66680 100644
> --- a/arch/powerpc/include/asm/system.h
> +++ b/arch/powerpc/include/asm/system.h
> @@ -221,6 +221,7 @@ extern unsigned long klimit;
>  extern void *zalloc_maybe_bootmem(size_t size, gfp_t mask);
>  
>  extern int powersave_nap;	/* set if nap mode can be used in idle loop */
> +void cpu_idle_wait(void);
>  
>  /*
>   * Atomic exchange
> diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
> index 39a2baa..b478c72 100644
> --- a/arch/powerpc/kernel/idle.c
> +++ b/arch/powerpc/kernel/idle.c
> @@ -39,9 +39,13 @@
>  #define cpu_should_die()	0
>  #endif
>  
> +unsigned long boot_option_idle_override = IDLE_NO_OVERRIDE;
> +EXPORT_SYMBOL(boot_option_idle_override);
> +
>  static int __init powersave_off(char *arg)
>  {
>  	ppc_md.power_save = NULL;
> +	boot_option_idle_override = IDLE_POWERSAVE_OFF;
>  	return 0;
>  }
>  __setup("powersave=off", powersave_off);
> @@ -102,6 +106,28 @@ void cpu_idle(void)
>  	}
>  }
>  
> +
> +/*
> + * cpu_idle_wait - Used to ensure that all the CPUs come out of the old
> + * idle loop and start using the new idle loop.
> + * Required while changing idle handler on SMP systems.
> + * Caller must have changed idle handler to the new value before the call.
> + */
> +void cpu_idle_wait(void)
> +{
> +	int cpu;
> +	smp_mb();
> +
> +	/* kick all the CPUs so that they exit out of old idle routine */
> +	get_online_cpus();
> +	for_each_online_cpu(cpu) {
> +		if (cpu != smp_processor_id())
> +			smp_send_reschedule(cpu);
> +	}
> +	put_online_cpus();
> +}
> +EXPORT_SYMBOL_GPL(cpu_idle_wait);
> +
>  int powersave_nap;
>  
>  #ifdef CONFIG_SYSCTL
> 
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