[lttng-dev] Tracing/Profiling boot

[Mathieu Desnoyers]

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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