[lttng-dev] Tracing/Profiling boot

[Martin Townsend]

Hi Mathieu,

Thanks for the info, I didn't know about ftrace, I will take a look at
this.

Out of interest would there be any benefits from using a built-in lttng
module over ftrace?

Cheers,
Martin.

On Thu, Feb 25, 2016 at 9:18 PM, Mathieu Desnoyers <
mathieu.desnoyers at efficios.com> wrote:

> Hi Martin,
>
> The main limitation LTTng currently has for early boot tracing is that
> you need to first spawn a lttng-sessiond user-space process, and setup
> tracing, before you can actually do any tracing. As long as you can
> fit within those constraints, you should be OK.
>
> If you really want to trace earlier than that, you might have to create
> a dedicated early-boot tracing module that would setup tracing
> buffers into a "dummy" session which exists only within lttng-modules,
> and then allow sessiond to later hook on those buffers when user-space
> is ready. Nothing exists for this at the moment. Note that since
> lttng-modules master (upcoming 2.8), you can now build lttng-modules
> into your kernel image, this might be useful for you. See the "kernel
> built-in support" section in
> https://github.com/lttng/lttng-modules/blob/master/README.md
>
> Since LTTng 2.0, we have left early boot tracing to other tools, such
> as Ftrace, which target kernel developers use-cases, and focused
> more on tracing of the system in its execution phases which are more
> relevant to application developers.
>
> If you want to go ahead and create a LTTng modules module that
> allow early boot tracing, I'd be happy to provide ideas and review.
>
> Thanks,
>
> Mathieu
>
> ----- On Feb 25, 2016, at 3:56 PM, Martin Townsend <
> mtownsend1973 at gmail.com> wrote:
>
> Hi,
>
> This is a bit of a long shot but does LTTng allow you trace boot?
>
> I'm seeing a weird problem where if I boot with systemd-bootchart if boots
> faster than just using systemd as the init process.  I created my own init
> process based on  systemd-bootchart and worked out it was down to the fact
> it called nanosleep, so I now have my own init process which hands over to
> systemd and creates a child that nanosleeps for the boot duration.  I would
> really like to trace/profile the scheduler and hrtimers understand what's
> happening and try and get a proper fix :)  Even if it means a bit of
> hacking kernel/LTTng, I would be willing to do this.
>
> Many Thanks, Martin.
>
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>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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