linux-kernel - Re: [PATCH v3] arch_topology: Trace the update thermal pressure

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Date:   Wed, 27 Apr 2022 09:01:58 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, sudeep.holla@....com,
        dietmar.eggemann@....com, vincent.guittot@...aro.org,
        rafael@...nel.org, rostedt@...dmis.org, mingo@...hat.com
Subject: Re: [PATCH v3] arch_topology: Trace the update thermal pressure



On 4/27/22 08:58, Greg KH wrote:
> On Wed, Apr 27, 2022 at 08:52:50AM +0100, Lukasz Luba wrote:
>>
>>
>> On 4/27/22 08:44, Greg KH wrote:
>>> On Wed, Apr 27, 2022 at 08:35:51AM +0100, Lukasz Luba wrote:
>>>> Add trace event to capture the moment of the call for updating the thermal
>>>> pressure value. It's helpful to investigate how often those events occur
>>>> in a system dealing with throttling. This trace event is needed since the
>>>> old 'cdev_update' might not be used by some drivers.
>>>>
>>>> The old 'cdev_update' trace event only provides a cooling state
>>>> value: [0, n]. That state value then needs additional tools to translate
>>>> it: state -> freq -> capacity -> thermal pressure. This new trace event
>>>> just stores proper thermal pressure value in the trace buffer, no need
>>>> for additional logic. This is helpful for cooperation when someone can
>>>> simply sends to the list the trace buffer output from the platform (no
>>>> need from additional information from other subsystems).
>>>>
>>>> There are also platforms which due to some design reasons don't use
>>>> cooling devices and thus don't trigger old 'cdev_update' trace event.
>>>> They are also important and measuring latency for the thermal signal
>>>> raising/decaying characteristics is in scope. This new trace event
>>>> would cover them as well.
>>>>
>>>> We already have a trace point 'pelt_thermal_tp' which after a change to
>>>> trace event can be paired with this new 'thermal_pressure_update' and
>>>> derive more insight what is going on in the system under thermal pressure
>>>> (and why).
>>>>
>>>> Reported-by: kernel test robot <lkp@...el.com>
>>>
>>> The kernel test robot did not report that you needed to add a new trace
>>> event :(
>>>
>>
>> I got feedback from the test robot for v1, which figured out that
>> the riscv configuration is broken. You can find it here
>> https://lore.kernel.org/lkml/202204201654.vcszVDGb-lkp@intel.com/
>>
>> So, I've added that tag following:
>> "If you fix the issue, kindly add following tag as appropriate"
>>
>> Should this only be honored when a patch actually got into next
>> and then following patch with a fix would have that tag?
> 
> Yes.  And you can mention it in the version information about what
> changed between each patch version below the --- line, but as is, you
> can see how this does not make sense.
> 

Thank you Greg for the explanation. I'll remove the tag in v4.

Regards,
Lukasz