Hi Naveen,

On Tue, Feb 04, 2020 at 01:22:19PM +0530, Naveen N. Rao wrote:

Gautham R Shenoy wrote:

quoted

Hi Naveen,

On Thu, Dec 05, 2019 at 10:23:58PM +0530, Naveen N. Rao wrote:

quoted

quoted

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 ?

Since it is possible for a cpu to go into extended cede for multiple seconds
during which time it is possible to mis-report utilization, I think it is
better to ensure that the sysfs interface for idle_[s]purr report the proper
values through use of IPI.

Fair enough.

With repect to lparstat, the read interval is user-specified and just gets
passed onto sleep().

Ok. So I guess currently you will be sending smp_call_function every
time you read a PURR and SPURR. That number will now increase by 2
times when we read idle_purr and idle_spurr.

- Naveen

--
Thanks and Regards
gautham.