linux-kernel - Re: [PATCH v4] arch_topology: Trace the update thermal pressure

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Date:   Wed, 27 Apr 2022 10:21:44 +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 v4] arch_topology: Trace the update thermal pressure



On 4/27/22 10:20, Greg KH wrote:
> On Wed, Apr 27, 2022 at 09:08:06AM +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).
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> 
> Much better, thanks.  I'll wait for Sudeep's comments/review before
> taking it into my tree.
> 
> greg k-h

Thank you Greg!