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

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Date:   Fri, 2 Mar 2018 18:50:18 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     "Du, Changbin" <changbin.du@...el.com>,
        "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 Fri, Mar 02, 2018 at 11:39:16AM +0100, Rafael J. Wysocki wrote:
> 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?
>
yeah, I can guarantee all in kernel tools updated but people's private script.
For me, I just read the raw event for debug purpose. It is fair enough that leave
code as it was considering users' private tool based on this event.

> 
> 

-- 
Thanks,
Changbin Du