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

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Date:   Thu, 05 Dec 2019 10:16:19 -0600
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

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.