Date:	Tue, 27 Sep 2011 11:42:06 -0700
From:	Simon Glass <sjg@...omium.org>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/3] Add accurate boot timing to a Linux system

Hi Tim,

On Tue, Sep 27, 2011 at 10:56 AM, Tim Bird <tim.bird@...sony.com> wrote:
> On 09/23/2011 04:03 PM, Simon Glass wrote:
>> This experimental patch set adds boot timing to a Linux system. The
>> timing starts with the boot loader and extends through the kernel into
>> user space to the completion of the boot process. The timing starts when
>> the system leaves reset, not later when the kernel starts.
>
> I would be very interested in this.  This is something that
> would be very helpful, I believe, to assist with optimizing
> overall boot time.

Good.

>
>> Finally, in user space there is no kernel-blessed way to record
>> timestamps. One approach is to add lines to the init scripts like
>> 'cat /proc/uptime >/tmp/login_starts'.
>> This creates another place where
>> the boot timing tool must look for information.
>
> 'cat /proc/uptime >/dev/kmsg' (with printk timestamps on) is much
> better than the above, for this reason.

Yes, although this needs root access on my machine. Am hoping to have
something easily accessible from user space.

>
>> This Patchset
>> -------------
>> This patchset aims to unify timing in one place: a simple driver which
>> collects pre-kernel boot timestamps, adds its own as it boots, with
>> calls to bootstage_mark(), then allows user space (init, etc.) to add
>> more with 'echo "message" >>/sys/kernel/debug/bootstage/mask'.
>>
>> Finally it permits user space to access the full list of timestamps
>> with 'cat /sys/kernel/debug/bootstage/report', which has two columns:
>> the stage name and the timestamp:
>>
>>       reset   0
>>       arch_cpu_init-AVP       258902
>>       arch_cpu_init-A9        263267
>>       arch_cpu_init-done      263312
>>       board_init_f-start      263314
>>       board_init_r-start      323671
>>       main_loop       573008
> ...
>
> I would prefer the timestamp in the first column.  Also, for consistency
> it would be good if it used the same format as printk timestamps:
> "[%5lu.%06lu]" with seconds and micro-seconds in the respective fields.
> Then, existing tools like scripts/show_delta and scripts/bootgraph.pl
> could work on this data as well.

Yes that makes sense, I will adjust it.

>
> Full micro-second granularity is not required, but it's nice to keep
> the format the same, whether the clock supports it or not.

Yes, and it's surprising how often microsecond accuracy is useful. For
example, recently I was looking at the impact of reduced
voltage/frequency early in boot on an ARM platform. The difference was
only a few ms, and the microsecond timer resolution helped a lot in
seeing the impact of these changes.

>
> Finally, is this work related at all to this:
> http://lists.denx.de/pipermail/u-boot/2011-September/099996.html
> Bootgraph.pl instrumentation support for UBoot
> ??
>
> Just wondering.

It is related in the sense that ideally all of this would come
together and play nicely. That's an interesting patch also, and there
are a few threads to tie together in U-Boot independently of the
kernel.

My original U-Boot patch set was here:

http://lists.denx.de/pipermail/u-boot/2011-May/092584.html

>
> Thanks - this looks like great stuff!

Thanks!

Regards,
Simon

>  -- Tim
>
> =============================
> Tim Bird
> Architecture Group Chair, CE Workgroup of the Linux Foundation
> Senior Staff Engineer, Sony Network Entertainment
> =============================
>
>
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