Re: [RFC][BUG] tracer: Fails to work

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jan 28, 2016, at 9:47 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Thu, Jan 28, 2016 at 02:38:16PM +0000, Mathieu Desnoyers wrote:
>> ----- On Jan 28, 2016, at 8:57 AM, Peter Zijlstra peterz@infradead.org wrote:
>> 
>> > On Thu, Jan 28, 2016 at 01:38:00PM +0000, Mathieu Desnoyers wrote:
>> > 
>> >> Thoughts ?
>> > 
>> > So ideally dumping the trace data would not depend on any of that,
>> > because I can break it all :-)
>> > 
>> > Not being able to access the trace data completely and utterly defeats
>> > the purpose of having a tracer in the first place.
>> 
>> One item I have on my todo list is to allow mapping
>> the tracer buffers (lttng in my case) onto RAM that
>> persists across reboots/kexec using dax and the pmem
>> driver. Since the original system is clearly inactive
>> after a reboot, we can read the buffers from memory
>> without caring about synchronization.
> 
> I would expect the new kernel to use the same buffer for its tracing,
> right? After all, there's no distinction between the old and new kernel
> except a reboot.

The new kernel can allocate the buffers into separate "files"
on the DAX in-RAM filesystem, so the old buffers would still be
there even when running the new kernel.

> 
> In which case, my kernels often start babbling the moment I boot. So I
> need to take care not to scribble the old data.
> 
>> That would be one possible way of handling your
>> snapshot-of-buggy-kernel-trace-buffers use-case.
> 
> Now I wonder if my ipmipower -r is a warn reset :-)
> 
> (I'm assuming the pmem/dax stuff would simply use regular RAM to back
> this stuff)

On x86, many BIOS wipe the memory content even after a warm reset :(
The most reliable solution I found on x86 is to use kexec() to
boot into a new kernel, mount the DAX filesystem again and read the
buffers from there.

I currently have the persistent buffer support implemented for the
lttng userspace tracer, but not yet for the kernel tracer. It's high
on my todo priority list though :)

Thanks,

Mathieu

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