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

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Message-Id: <1575566328.nhfi897fmd.naveen@linux.ibm.com>
Date:   Thu, 05 Dec 2019 22:55:05 +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

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.


- Naveen