linux-kernel - Re: [PATCH v2] tracing/power: Polish the tracepoints cpu_idle and cpu

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Date:   Fri, 2 Mar 2018 11:39:16 +0100
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     "Du, Changbin" <changbin.du@...el.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tracing/power: Polish the tracepoints cpu_idle and
 cpu_frequency

On 3/2/2018 11:15 AM, Du, Changbin wrote:
> On Fri, Mar 02, 2018 at 11:18:10AM +0100, Rafael J. Wysocki wrote:
>> On Fri, Mar 2, 2018 at 10:41 AM, Du, Changbin <changbin.du@...el.com> wrote:
>>>>>> That rather isn't the case if negative values are ever passed to the
>>>>>> tracepoint, right?
>>>>>>
>>>>> yes.
>>>>>> Which seems to be the reason why you want to make this change, isn't it?
>>>>>>
>>>>> yes, to improve readability.
>>>>>
>>>>>> So maybe fix the code using the tracepoint(s) to avoid passing
>>>>>> negative values to it(them)?
>>>>> For cpu_idle event, [0, CPUIDLE_STATE_MAX) are used to index the idle state arrary,
>>>>> so I think a appropriate value for PWR_EVENT_EXIT is -1 (defined in include/trace/events/power.h).
>>>>> Or do you have a better idea? Thanks!
>>>> Sorry, I'm not sure what you mean.
>>>>
>>>> I'm saying that the code using the CPU PM tracepoints is not expected
>>>> to pass -1 as the CPU number to them.  IOW, neither -1 nor its UL
>>>> representation should ever appear in the output of these tracepoints.
>>>> If that happens, it is a problem with the code using the tracepoints
>>>> which needs to be fixed.  Users should not see any of these values.
>>> This patch only changed 'state' field but cpuid. For cpu_idle event, 'state' is
>>> singned value, but for cpu_frequency it is unsinged.
>>> The cpuid is always unsinged value. So no one passes -1 as CPU number.
>> You are right, 'state' not 'cpuid', sorry.
>>
>> Negative 'state' should not be passed to these tracepoints too, though.
> The current situtation is that 'state' can be negative for event cpu_idle :(. This
> is why I made this change.
>
And which is why I said that IMO it would be better to change the 
current situation.

Your patch makes the results of it slightly less confusing to a human 
reader of the tracepoint output, but the situation is still unchanged 
after it.

And what if someone has a script built around these tracepoints that 
knows how to handle the UL representation of -1, but doesn't know how to 
parse "-1"?  They would need to update the script after your change, 
wouldn't they?  And why would it be OK to inflict that work on them just 
to improve the readability of the output for humans?