[ltt-dev] [PATCH v2] add tracepoints of panic and kexec

[Lai Jiangshan]

Mathieu Desnoyers wrote:
> * Zhaolei (zhaolei at cn.fujitsu.com) wrote:
>> * From: "Mathieu Desnoyers" <compudj at krystal.dyndns.org>
>>> * Zhaolei (zhaolei at cn.fujitsu.com) wrote:
>>>> * From: "Mathieu Desnoyers" <compudj at krystal.dyndns.org>
>>>>> * Zhaolei (zhaolei at cn.fujitsu.com) wrote:
>>>>>> Instrumentation of following panic and kexec related events are added:
>>>>>> panic
>>>>>> kernel_kexec
>>>>>> crash_kexec
>>>>>>
>>>>>> It is useful for build flight-recorder program based on lttng infrastructure.
>>>>>>
>>>>> Hrm, I'm wondering if these are events we are interested to trace or if
>>>>> you plan to use these tracepoints as hooks into the kernel to connect
>>>>> the tracer infrastructure to it ?
>>>> Hello, Mathieu
>>>>
>>>> IMHO, both.
>>>>
>>>> As a programmer who want to build  flight-recorder based on lttng infrastructure,
>>>> he can:
>>>> 1: use lttng trace, and get lttng's trace datas from kernel-dump file.
>>>> or
>>>> 2: use lttng's tracepoint, and record events himself.
>>>>
>>>> But as a lttng's programmer, we can make both way possible.
>>>>
>>>> What is your opinion?
>>>>
>>> Yes, I think having both is good. So we could add, separately :
>>>
>>> A hook in panic and kexec using include/linux/notifier.h so mechanisms
>>> other than tracers can connect their hook to it.
>>>
>>> Also add a tracepoint so tracers can use this as an event source.
>>>
>>> I would do both in separate patches though, because they have different
>>> purposes.
>> Hello, Mathieu
>>
>> Are you means you will do both in next version of lttng?
>> Thanks!
>>
> 
> Hi Zhaolei,
> 
> I won't create the patches myself because I don't see it as a priority
> (Linus said he would refuse any instrumentation patch before we get the
> data output mechanism into the kernel), but if someone provides patches

Hi, Mathieu,

You *forgot* one thing: we are lttng *users*, not only developers.
I don't think it's good that you just heard the God only.

lttng is used by her users at the end. User's need is one of the
first class things to concern.

> to add notifier feature to kexec and panic so we can plug LTTng into
> them to extract traces from a crashed kernel, I would be very interested
> to add this. OTOH, about the tracepoints, I am not convinced they are
> necessary for now, because we already instrument printk which logs some
> information on panic(). Maybe kexec would be better suited for
> instrumentation.

We need to know the time and other information that panic happened.
And output message is integrated in lttng with other messages.
I don't think it's bad idea which we use for a long time.

And this is a *key message*, event should be generated efficient/elegant.

Sorry for participated in this discuss with bad manners.

Thanks, Lai.

> 
> Also, it would be good to provide the LTTng probes for the tracepoints
> you add in a separate patch within the same patchset
> (ltt/probes/*-trace.c).
> 
> Thanks,
> 
> Mathieu
>