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    SubjectRe: [PATCH] power, trace: add tracing for device_resume
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    On Saturday, May 19, 2012, Sameer Nanda wrote:
    > On Fri, May 18, 2012 at 4:01 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
    > > On Fri, May 18, 2012 at 03:17:32PM -0700, Sameer Nanda wrote:
    > >> On Fri, May 18, 2012 at 2:46 PM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
    > >> > On Friday, May 18, 2012, Sameer Nanda wrote:
    > >> >> On Fri, May 18, 2012 at 2:19 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
    > >> >> > On Fri, 2012-05-18 at 13:58 -0700, Sameer Nanda wrote:
    > >> >> >> On Fri, May 18, 2012 at 12:14 PM, Steven Rostedt <rostedt@goodmis.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.
    > >> >> >
    > >> >> > Just note that every TRACE_EVENT() adds around 5k or more code. Every
    > >> >> > DEFINE_EVENT adds just about 300 bytes.
    > >> >>
    > >> >> Ok, let me respin the patch. I am thinking of adding time_delta to
    > >> >> all three traces. That way we should get the space saving while still
    > >> >> allowing quick spotting of devices that take long time to resume.
    > >> >
    > >> > Well, what's wrong with the code in drivers/base/power/main.c that
    > >> > is activated by adding initcall_debug to the kernel command line?
    > >>
    > >> Mostly that I hadn't looked closely at initcall_debug before writing my patch :)
    > >>
    > >> Now that I have taken a look at it, the main issue is that the kernel
    > >> command line needs to be modified to activate it. We cannot do this
    > >> for our automated regression suites since the kernel command line is
    > >> protected on Chrome OS systems.
    > >
    > > You are kidding, right? You have control over your test systems, don't
    > > bloat everyone's kernel by 5k just because your infrastructure is
    > > somehow something that you feel you can't change.
    >
    > Fair enough. But having to modify the kernel command line to do this
    > is clunky. How about exposing the ability to turn on these
    > initcall_debug prints through a knob under /sys/power?

    This might work, but first you'd need to make them depend on something
    different from initcall_debug (and make that thing in turn be set if
    initcall_debug is put into the kernel command line). Then, you could
    export the new variable.

    Greg, does that make sense to you?

    Rafael


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