linux-kernel - Re: [PATCH v2] timer stats: add a 'status' line to timer usage statistics

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Date:	Thu, 10 Oct 2013 08:16:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dong Zhu <bluezhudong@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] timer stats: add a 'status' line to timer usage
 statistics


* Dong Zhu <bluezhudong@...il.com> wrote:

> Hi Ingo and John,
> 
> On Thu, Oct 10, 2013 at 06:35:19AM +0200, 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.
> 
> Yes, I am wrong about it. Thanks for pointing this out and pretty sorry
> for confusing you John.
> 
> > 
> > Instead of this change I'd suggest adding a 'status' line, with two 
> > outputs:
> > 
> > 	Status: collection active
> > 
> > 	Status: collection disabled
> > 
> 
> Agree, I modified this patch and resubmited it again, Could you help
> reviewing it again ? Thanks !
> 
> From 263c40abea8011c82582b2d671ae783b26f44bd5 Mon Sep 17 00:00:00 2001
> From: Dong Zhu <bluezhudong@...il.com> 
> Date: Thu, 10 Oct 2013 13:46:08 +0800
> 
> We can enable|disable timer statistics collection (via echo [1|0] >
> /proc/timers_stats), this patch adds a 'status' line to display the
> current timer collection status.
> 
> Signed-off-by: Dong Zhu <bluezhudong@...il.com>
> ---
>  kernel/time/timer_stats.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
> index 0b537f2..bac5e91 100644
> --- a/kernel/time/timer_stats.c
> +++ b/kernel/time/timer_stats.c
> @@ -303,6 +303,10 @@ static int tstats_show(struct seq_file *m, void *v)
>  	if (atomic_read(&overflow_count))
>  		seq_printf(m, "Overflow: %d entries\n",
>  			atomic_read(&overflow_count));
> +	if (timer_stats_active)
> +		seq_puts(m, "Status: collection active\n");
> +	else
> +		seq_puts(m, "Status: collection disabled\n");

I suspect we could do something like:

	seq_printf("Status: collection %s\n", timer_stats_active ? "enabled" : "disabled");

and save a bit of kernel image size?

Also, please bump up the version to v0.3, to give parsers a chance.

Otherwise it looks good to me.

Thanks,

	Ingo
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