linux-kernel - Re: [PATCH 6/7] thermal: netlink: Add a new event to notify CPU capabilities change

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Message-ID: <a2389c0f-c42d-0198-a625-0eb2a97628f0@arm.com>
Date:   Tue, 9 Nov 2021 17:51:02 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-pm@...r.kernel.org, x86@...nel.org,
        linux-doc@...r.kernel.org, Len Brown <len.brown@...el.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Amit Kucheria <amitk@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] thermal: netlink: Add a new event to notify CPU
 capabilities change



On 11/9/21 2:15 PM, Srinivas Pandruvada wrote:
> On Tue, 2021-11-09 at 13:53 +0000, Lukasz Luba wrote:
>> Hi Srinivas,
>>
>> On 11/9/21 1:23 PM, Srinivas Pandruvada wrote:
>>> Hi Lukasz,
>>>
>>> On Tue, 2021-11-09 at 12:39 +0000, Lukasz Luba wrote:
>>>> Hi Ricardo,
>>>>
>>>>
>>>> On 11/6/21 1:33 AM, Ricardo Neri wrote:
>>>>> From: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
>>>>>
>>>>> Add a new netlink event to notify change in CPU capabilities in
>>>>> terms of
>>>>> performance and efficiency.
>>>>
>>>> Is this going to be handled by some 'generic' tools? If yes,
>>>> maybe
>>>> the values for 'performance' might be aligned with capacity
>>>> [0,1024] ? Or are they completely not related so the mapping is
>>>> simply impossible?
>>>>
>>>
>>> That would have been very useful.
>>>
>>> The problem is that we may not know the maximum performance as
>>> system
>>> may be booting with few CPUs (using maxcpus kernel command line)
>>> and
>>> then user hot adding them. So we may need to rescale when we get a
>>> new
>>> maximum performance CPU and send to user space.
>>>
>>> We can't just use max from HFI table at in instance as it is not
>>> necessary that HFI table contains data for all CPUs.
>>>
>>> If HFI max performance value of 255 is a scaled value to max
>>> performance CPU value in the system, then this conversion would
>>> have
>>> been easy. But that is not.
>>
>> I see. I was asking because I'm working on similar interface and
>> just wanted to understand your approach better. In my case we
>> would probably simply use 'capacity' scale, or more
>> precisely available capacity after subtracting 'thermal pressure'
>> value.
>> That might confuse a generic tool which listens to these socket
>> messages, though. So probably I would have to add a new
>> THERMAL_GENL_ATTR_CPU_CAPABILITY_* id
>> to handle this different normalized across CPUs scale.
> I can add a field capacity_scale. In HFI case it will always be 255. In
> your cases it will 1024.
> 
> 

Sounds good, with that upper limit those tools would not build
up assumptions (they would have to parse that scale value).
Although, I would prefer to call it 'performance_scale' if you don't
mind.
I've done similar renaming  s/capacity/performance/ in the Energy Model
(EM) some time ago [1]. Some reasons:
- in the scheduler we have 'Performance Domains (PDs)'
- for GPUs we talk about 'performance', because 'capacity' sounds odd
   in that case

[1] 
https://lore.kernel.org/linux-pm/20200527095854.21714-2-lukasz.luba@arm.com/