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

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Message-Id: <48823589-b105-0da3-e532-f633ade8f0d9@linux.vnet.ibm.com>
Date:   Thu, 5 Dec 2019 20:33:23 +0530
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     Nathan Lynch <nathanl@...ux.ibm.com>,
        "Gautham R. Shenoy" <ego@...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>
Subject: Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

On 12/5/19 3:54 AM, Nathan Lynch wrote:
> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
>> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>>
>> On PSeries LPARs, the data centers planners desire a more accurate
>> view of system utilization per resource such as CPU to plan the system
>> capacity requirements better. Such accuracy can be obtained by reading
>> PURR/SPURR registers for CPU resource utilization.
>>
>> 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.

-- 
Kamalesh