linux-kernel - Re: [PATCH RESEND] timer stats: reset entries when disable the timer usage statistics

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Date:	Wed, 09 Oct 2013 21:48:03 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Ingo Molnar <mingo@...nel.org>, Dong Zhu <bluezhudong@...il.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] timer stats: reset entries when disable the timer
 usage statistics

On 10/09/2013 09:35 PM, Ingo Molnar wrote:
> * Dong Zhu <bluezhudong@...il.com> wrote:
>
>> From f41628c61d8a9172677ba33a55b61e37ce28f7a6 Mon Sep 17 00:00:00 2001
>> From: Dong Zhu <bluezhudong@...il.com> 
>> Date: Thu, 10 Oct 2013 10:38:13 +0800
>>
>> When we stop timer statistics collection (via echo 0 > 
>> /proc/timers_stats), the statistics data is still exported as if it were 
>> correct, which can cause applicaitons to misuse the statistics.
> What misuse do you mean?
>
>> This patch resets the statistics when we stop collecting them, to avoid 
>> this problem.
> Well, this loses the handy 'snapshot' property of /proc/timer_stats. 
> Before this change one could do:
>
> 	echo 1 > /proc/timers_stats
> 	sleep 60 # run system workload
> 	echo 0 > /proc/timers_stats
>
> and examine the 1-minute collection result without it changing. Your 
> change, if I understand it correctly, zeroes it all out.


Good point. I misunderstood the patch and thought it was an issue that
the statistics are never cleared even if the collection was disabled,
but looking closer I see we clear on enable (if we were disabled). So
yes, this change doesn't seem right.



> Instead of this change I'd suggest adding a 'status' line, with two 
> outputs:
>
> 	Status: collection active
>
> 	Status: collection disabled

Agreed.

thanks
-john
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