Subject: Re: [PATCH v2 4/5] powerpc/sysfs: Show idle_purr and idle_spurr for every CPU

From: Nathan Lynch <[email-masked]>
Date: Fri, 21 Feb 2020 10:50:12 -0600

"Gautham R. Shenoy" <[email-masked]> writes:
> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> index 80a676d..5b4b450 100644
> --- a/arch/powerpc/kernel/sysfs.c
> +++ b/arch/powerpc/kernel/sysfs.c
> @@ -19,6 +19,7 @@
>  #include <asm/smp.h>
>  #include <asm/pmc.h>
>  #include <asm/firmware.h>
> +#include <asm/idle.h>
>  #include <asm/svm.h>
>  
>  #include "cacheinfo.h"
> @@ -733,6 +734,42 @@ static void create_svm_file(void)
>  }
>  #endif /* CONFIG_PPC_SVM */
>  
> +static void read_idle_purr(void *val)
> +{
> +	u64 *ret = (u64 *)val;

No cast from void* needed.

> +
> +	*ret = read_this_idle_purr();
> +}
> +
> +static ssize_t idle_purr_show(struct device *dev,
> +			      struct device_attribute *attr, char *buf)
> +{
> +	struct cpu *cpu = container_of(dev, struct cpu, dev);
> +	u64 val;
> +
> +	smp_call_function_single(cpu->dev.id, read_idle_purr, &val, 1);
> +	return sprintf(buf, "%llx\n", val);
> +}
> +static DEVICE_ATTR(idle_purr, 0400, idle_purr_show, NULL);
> +
> +static void read_idle_spurr(void *val)
> +{
> +	u64 *ret = (u64 *)val;
> +
> +	*ret = read_this_idle_spurr();
> +}
> +
> +static ssize_t idle_spurr_show(struct device *dev,
> +			       struct device_attribute *attr, char *buf)
> +{
> +	struct cpu *cpu = container_of(dev, struct cpu, dev);
> +	u64 val;
> +
> +	smp_call_function_single(cpu->dev.id, read_idle_spurr, &val, 1);
> +	return sprintf(buf, "%llx\n", val);
> +}
> +static DEVICE_ATTR(idle_spurr, 0400, idle_spurr_show, NULL);

It's regrettable that we have to wake up potentially idle CPUs in order
to derive correct idle statistics for them, but I suppose the main user
(lparstat) of these interfaces already is causing this to happen by
polling the existing per-cpu purr and spurr attributes.

So now lparstat will incur at minimum four syscalls and four IPIs per
CPU per polling interval -- one for each of purr, spurr, idle_purr and
idle_spurr. Correct?

At some point it's going to make sense to batch sampling of remote CPUs'
SPRs.

>  static int register_cpu_online(unsigned int cpu)
>  {
>  	struct cpu *c = &per_cpu(cpu_devices, cpu);
> @@ -794,10 +831,15 @@ static int register_cpu_online(unsigned int cpu)
>  		if (!firmware_has_feature(FW_FEATURE_LPAR))
>  			add_write_permission_dev_attr(&dev_attr_purr);
>  		device_create_file(s, &dev_attr_purr);
> +		if (firmware_has_feature(FW_FEATURE_LPAR))
> +			device_create_file(s, &dev_attr_idle_purr);
>  	}
>  
> -	if (cpu_has_feature(CPU_FTR_SPURR))
> +	if (cpu_has_feature(CPU_FTR_SPURR)) {
>  		device_create_file(s, &dev_attr_spurr);
> +		if (firmware_has_feature(FW_FEATURE_LPAR))
> +			device_create_file(s, &dev_attr_idle_spurr);
> +	}
>  
>  	if (cpu_has_feature(CPU_FTR_DSCR))
>  		device_create_file(s, &dev_attr_dscr);
> @@ -879,11 +921,17 @@ static int unregister_cpu_online(unsigned int cpu)
>  	if (cpu_has_feature(CPU_FTR_MMCRA))
>  		device_remove_file(s, &dev_attr_mmcra);
>  
> -	if (cpu_has_feature(CPU_FTR_PURR))
> +	if (cpu_has_feature(CPU_FTR_PURR)) {
>  		device_remove_file(s, &dev_attr_purr);
> +		if (firmware_has_feature(FW_FEATURE_LPAR))
> +			device_remove_file(s, &dev_attr_idle_purr);
> +	}
>  
> -	if (cpu_has_feature(CPU_FTR_SPURR))
> +	if (cpu_has_feature(CPU_FTR_SPURR)) {
>  		device_remove_file(s, &dev_attr_spurr);
> +		if (firmware_has_feature(FW_FEATURE_LPAR))
> +			device_remove_file(s, &dev_attr_idle_spurr);
> +	}
>  
>  	if (cpu_has_feature(CPU_FTR_DSCR))
>  		device_remove_file(s, &dev_attr_dscr);

The cpu register/unregister stuff here looks correct.