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

From: Gautham R Shenoy <[email-masked]>
Date: Wed, 5 Feb 2020 09:49:56 +0530

Hi Naveen,

On Tue, Feb 04, 2020 at 01:22:19PM +0530, Naveen N. Rao wrote:
> Gautham R Shenoy wrote:
> >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 ?
> 
> 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.