linux-kernel - Re: [PATCH 3/3] trace points: power: remove 'cpu_id' from trace_cpu

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Date:	Fri, 2 Sep 2011 09:26:28 +0200
From:	Jean Pihet <jean.pihet@...oldbits.com>
To:	Thomas Renninger <trenn@...e.de>, Ming Lei <tom.leiming@...il.com>
Cc:	rostedt@...dmis.org, fweisbec@...il.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 3/3] trace points: power: remove 'cpu_id' from trace_cpu_idle

Ming Lei, Thomas,

Sorry if it is a bit late to jump in.

On Mon, Aug 22, 2011 at 10:27 AM, Thomas Renninger <trenn@...e.de> wrote:
> On Saturday, August 20, 2011 04:40:09 AM Ming Lei wrote:
>> Hi,
>>
>> 2011/8/20 Thomas Renninger <trenn@...e.de>:
>> > On Friday, August 19, 2011 05:04:04 PM tom.leiming@...il.com wrote:
>> >> From: Ming Lei <tom.leiming@...il.com>
>> >>
>> >> This patch removes the 'cpu_id' parameter of the cpu_idle
>> >> trace point, based on the ideas below:
>> >>
>> >> - the cpu_id which is passed to trace point is always the current
>> >>   cpu
>> > Are you sure this will always be true?
>>
>> It is sure at least now,  the only place to pass 'dev->cpu' is inside
>> cpuidle_idle_call,
> It was known that cpu_id is always the current cpu with current
> implementation when this got introduced.
> But the perf events API must not change back and forth for userspace
> compatibility. Therefore the cpu_id was added in case
> that future implementations want to pass info where the current cpu
> is not the cpu which is sent to the sleep state.
Agree. Let's keep the cpu_id field.

>
>> smp_processor_id() can't be used safely in preemptible context.
> I expect the only side effect that could happen is that if smp_process_id
> is interrupted you get the wrong core id on a cpu idle trace event.
> This only happens if cpuidle is not used and even then should happen
> very rarely, nothing to worry for a debug tool like that.
> And it should get fixed if these idle functions get fully integrated into
> cpuidle at some point of time.
>
>   Thomas
>

Regards,
Jean
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