linux-kernel - Re: [PATCH] power, trace: add tracing for device

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Date:	Fri, 18 May 2012 13:58:09 -0700
From:	Sameer Nanda <snanda@...omium.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Greg KH <gregkh@...uxfoundation.org>, rob@...dley.net,
	len.brown@...el.com, pavel@....cz, rjw@...k.pl, fweisbec@...il.com,
	mingo@...hat.com, jkosina@...e.cz, standby24x7@...il.com,
	jj@...osbits.net, paulmck@...ux.vnet.ibm.com,
	josh@...htriplett.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] power, trace: add tracing for device_resume

On Fri, May 18, 2012 at 12:14 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2012-05-18 at 11:57 -0700, Sameer Nanda wrote:
>
>> AFAICT, they are used for something completely different -- help solve
>> suspend/resume issues by saving a hash in the RTC of the last device
>> that suspended/resumed.  They don't use the perf tracing mechanism at
>> all.
>>
>
> Also note that all tracepoints have timestamps attached to them. You do
> not need to add deltas. Do that in the userspace tools that read the
> timestamps and events. This way you can have one DECLARE_EVENT_CLASS and
> three DEFINE_EVENTs. This will save space.

Agreed on the space savings.  However, with the time_delta in the
trace message itself, a one line shell script [1] that sorts on the
time_delta field is sufficient to quickly spot the devices that take a
long time to resume.  Without the time_delta field, the user tool is
more complex since it needs to first match up the device_resume_in,
device_resume_waited and device_resume_out traces and then calculate
time deltas.

Seems like a worthwhile trade-off to me but I can take out the
time_delta if the general consensus is otherwise.

[1]: here's an example script I use for sorting the device resume times:
cat /sys/kernel/debug/tracing/trace | grep device_resume_out | awk
'BEGIN { FS = "time_delta=" } ; { print $2 $0 }' | sort -n


>
> -- Steve
>
>



-- 
Sameer
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