Subject: Re: [PATCH 2/3] powerpc/sysfs: Show idle_purr and idle_spurr for every CPU

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 3 Feb 2020 10:20:14 +0530

Hi Naveen,

On Thu, Dec 05, 2019 at 10:23:58PM +0530, Naveen N. Rao wrote:
> >diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> >index 80a676d..42ade55 100644
> >--- a/arch/powerpc/kernel/sysfs.c
> >+++ b/arch/powerpc/kernel/sysfs.c
> >@@ -1044,6 +1044,36 @@ static ssize_t show_physical_id(struct device *dev,
> > }
> > static DEVICE_ATTR(physical_id, 0444, show_physical_id, NULL);
> >
> >+static ssize_t idle_purr_show(struct device *dev,
> >+			      struct device_attribute *attr, char *buf)
> >+{
> >+	struct cpu *cpu = container_of(dev, struct cpu, dev);
> >+	unsigned int cpuid = cpu->dev.id;
> >+	struct lppaca *cpu_lppaca_ptr = paca_ptrs[cpuid]->lppaca_ptr;
> >+	u64 idle_purr_cycles = be64_to_cpu(cpu_lppaca_ptr->wait_state_cycles);
> >+
> >+	return sprintf(buf, "%llx\n", idle_purr_cycles);
> >+}
> >+static DEVICE_ATTR_RO(idle_purr);
> >+
> >+DECLARE_PER_CPU(u64, idle_spurr_cycles);
> >+static ssize_t idle_spurr_show(struct device *dev,
> >+			       struct device_attribute *attr, char *buf)
> >+{
> >+	struct cpu *cpu = container_of(dev, struct cpu, dev);
> >+	unsigned int cpuid = cpu->dev.id;
> >+	u64 *idle_spurr_cycles_ptr = per_cpu_ptr(&idle_spurr_cycles, cpuid);
> 
> Is it possible for a user to read stale values if a particular cpu is in an
> extended cede? Is it possible to use smp_call_function_single() to force the
> cpu out of idle?

Yes, if the CPU whose idle_spurr cycle is being read is still in idle,
then we will miss reporting the delta spurr cycles for this last
idle-duration. Yes, we can use an smp_call_function_single(), though
that will introduce IPI noise. How often will idle_[s]purr be read ?

> - Naveen

--
Thanks and Regards
gautham.