Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 04 Feb 2020 13:22:19 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH 2/3] powerpc/sysfs: Show idle_purr and idle_spurr for
 every CPU
To:     ego@linux.vnet.ibm.com
Cc:     Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nathan Lynch <nathanl@linux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@linux.ibm.com>
References: <1574856072-30972-1-git-send-email-ego@linux.vnet.ibm.com>
        <1574856072-30972-3-git-send-email-ego@linux.vnet.ibm.com>
        <1575564547.si4rk0s96p.naveen@linux.ibm.com>
        <20200203045013.GC13468@in.ibm.com>
In-Reply-To: <20200203045013.GC13468@in.ibm.com>
MIME-Version: 1.0
User-Agent: astroid/v0.15-13-gb675b421
 (https://github.com/astroidmail/astroid)
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8BIT
Message-Id: <1580802180.jpxk9s8apz.naveen@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

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

- Naveen