[lttng-dev] Tracing/Profiling boot

[Mathieu Desnoyers]

Hi Martin, 

I would say that the main advantages of using LTTng in this case 
would be on the tooling side, with availability of Trace Compass 
and LTTng Analyses projects to navigate in the resulting CTF 
traces, and get high-level overview of the various metrics that 
could slow down your system. 

LTTng would also allow you to correlate your kernel trace with 
a user-space trace gathered with lttng-ust, which gives you 
deeper insight into your applications. 

Thanks, 

Mathieu 

----- On Feb 26, 2016, at 3:41 AM, Martin Townsend <mtownsend1973 at gmail.com> wrote: 

> 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.

>>> _______________________________________________
>>> lttng-dev mailing list
>>> lttng-dev at lists.lttng.org
>>> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> http://www.efficios.com

-- 
Mathieu Desnoyers 
EfficiOS Inc. 
http://www.efficios.com 
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