linux-kernel - Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

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Message-Id: <1575623305.dgcux6u43j.naveen@linux.ibm.com>
Date:   Fri, 06 Dec 2019 14:44:06 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

Naveen N. Rao wrote:
> Hi Nathan,
> 
> Nathan Lynch wrote:
>> Hi Kamalesh,
>> 
>> Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com> writes:
>>> On 12/5/19 3:54 AM, Nathan Lynch wrote:
>>>> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
>>>>>
>>>>> Tools such as lparstat which are used to compute the utilization need
>>>>> to know [S]PURR ticks when the cpu was busy or idle. The [S]PURR
>>>>> counters are already exposed through sysfs.  We already account for
>>>>> PURR ticks when we go to idle so that we can update the VPA area. This
>>>>> patchset extends support to account for SPURR ticks when idle, and
>>>>> expose both via per-cpu sysfs files.
>>>> 
>>>> Does anything really want to use PURR instead of SPURR? Seems like we
>>>> should expose only SPURR idle values if possible.
>>>> 
>>>
>>> lparstat is one of the consumers of PURR idle metric
>>> (https://groups.google.com/forum/#!topic/powerpc-utils-devel/fYRo69xO9r4). 
>>> Agree, on the argument that system utilization metrics based on SPURR
>>> accounting is accurate in comparison to PURR, which isn't proportional to
>>> CPU frequency.  PURR has been traditionally used to understand the system
>>> utilization, whereas SPURR is used for understanding how much capacity is
>>> left/exceeding in the system based on the current power saving mode.
>> 
>> I'll phrase my question differently: does SPURR complement or supercede
>> PURR? You seem to be saying they serve different purposes. If PURR is
>> actually useful rather then vestigial then I have no objection to
>> exposing idle_purr.
> 
> SPURR complements PURR, so we need both. SPURR/PURR ratio helps provide 
> an indication of the available headroom in terms of core resources, at 
> maximum frequency.

Re-reading this today morning, I realize that this isn't entirely 
accurate. SPURR alone is sufficient to understand core resource 
utilization.

Kamalesh is using PURR to display non-normalized utilization values 
(under 'actual' column), as reported by lparstat on AIX. I am not 
entirely sure if it is ok to derive these based on the SPURR busy/idle 
ratio.

- Naveen