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

[Naveen N. Rao]

Gautham R Shenoy wrote:
> On Fri, Feb 21, 2020 at 10:50:12AM -0600, Nathan Lynch wrote:
> > "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx> 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?

Umm... procfs?
/me ducks

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

How did you mean this? It looks like we first need to provide a separate 
user interface, since with the existing sysfs interface providing 
separate files, I am not sure if we can batch such reads.


- Naveen