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

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 24 Feb 2020 10:44:47 +0530

On Fri, Feb 21, 2020 at 10:50:12AM -0600, Nathan Lynch wrote:
> "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.

Will fix this. Thanks.

> 
> 
> > +
> > +	*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?

Yes, it is unforunate that we will end up making four syscalls and
generating IPI noise, and this is something that I discussed with
Naveen and Kamalesh. We have the following two constraints:

1) These values of PURR and SPURR required are per-cpu. Hence putting
them in lparcfg is not an option.

2) sysfs semantics encourages a single value per key, the key being
the sysfs-file. Something like the following would have made far more
sense.

cat /sys/devices/system/cpu/cpuX/purr_spurr_accounting
purr:A
idle_purr:B
spurr:C
idle_spurr:D

There are some sysfs files which allow something like this. Eg: 
/sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state

Thoughts on any other alternatives?

> 
> 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.

Thanks for reviewing the patch.

--
Thanks and Regards
gautham.