[lttng-dev] Capturing snapshot on kernel panic

[Kienan Stewart]

Hi Damien,

I want to expand on one of the options that could work for your case.

On 5/16/24 9:37 AM, Kienan Stewart via lttng-dev wrote:
> Hi Damien,
> 
> 
> On 5/15/24 6:24 PM, Damien Berget via lttng-dev wrote:
>> Good day,
>> we have been using LTTng successfully to capture snapshots on user 
>> defined tracepoints and it did provide invaluable to debug our issues. 
>> Thanks to all the contributors of this project!
>>
>> We'd like to know if it would be possible to trigger on a kernel 
>> panic? I might be dubiously possible as you would still need to have 
>> the file-system working to write the results but I should ask.
>>
> 
> For userspace tracing, I think the recommendation is usually to use a 
> dax/pmem device and have the buffers for the session mapped there. After 
> a panic, the contents of the buffers can be restored using lttng-crash[1].
> 
> Note that dax/pem isn't supported by the kernel space tracer at this time.
> 
> If I recall, there are other ways to things in the panic sequence (that 
> aren't lttng specific), but I'm personally not as familiar with the 
> details of that stage of linux.
> 

It's possible to kexec-tools to load a new kernel post-panic[1]. If your 
system uses kexec, the contents of RAM aren't necessarily flushed, and 
if both the initial kernel and post-panic kernel started by kexec have 
the same configuration for an emulated PMEM device using the memmap 
paramenter [2,3] that region of memory can have a daxfs created in it 
post-clean boot.

Note: some systems may not flush the memory during a warm reboot, but 
this is dependent on the BIOS.

When your system boots you could do something like the following:

  * If it's a clean boot, create the daxfs
  * If it's an "unclean" boot (e.g. the daxfs already exists, or a 
kernel parameter informs you that it started post-panic) then you can 
copy/move/use lttng-crash to persistent storage for analysis
  * Start tracing using a snapshot session and the userspace buffers on 
the daxfs.

In this type of situation the "snapshot" command is never invoked 
directly, but the recovery of the buffers to create a snapshot is possible.

[1]: https://www.kernel.org/doc/html/latest/admin-guide/kdump/kdump.html
[2]: 
https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
[3]: 
https://docs.pmem.io/persistent-memory/getting-started-guide/creating-development-environments/linux-environments/linux-memmap

thanks,
kienan

>> Looking at available kernel syscall, the "reboot" one seems like a 
>> good candidate, however I was not able to capture a snapshot on it. I 
>> have tested the setup below with "--name=chdir" syscall and it 
>> works, "cd" to a directory will create a trace. But no dice with reboot.
>>
> 
> The details of how this work will depend on your system. For example, my 
> installations tend to use systemd as PID 1. The broad strokes seem to 
> be: `/usr/sbin/reboot` is actually a link to `systemctl`, which I 
> believe then kicks off the reboot.service, the PID 1 is swapped to 
> /usr/lib/systemd/systemd-shutdown, sigterm then sigkill are sent to all 
> processes, unmounts, syncs, calls the reboot system call [2,3].
> 
> As both the sigterm and the unmounts are done before the syscall, 
> lttng-sessiond and the consumers will have already shutdown by the time 
> it enters.
> 
> While this doesn't necessarily help your original question of panics, if 
> you want to snapshot before shutdown or reboot and are using systemd, 
> it's possible to leave a script or binary in a known directory so that 
> it's invoked prior to the rest of the shutdown sequence[4].
> 
> [1]: https://lttng.org/docs/v2.13/#doc-persistent-memory-file-systems
> [2]: 
> https://github.com/systemd/systemd/blob/6533c14997700f74e9ea42121303fc1f5c63e62b/src/shutdown/shutdown.c
> [3]: 
> https://github.com/systemd/systemd/blob/main/src/shared/reboot-util.c#L77
> [4]: https://www.systutorials.com/docs/linux/man/8-systemd-reboot/
> 
> hope this helps,
> kienan
> 
>> Would you have any suggestions?
>> Thanks for your help,
>> Cheers
>> Damien
>>
>> ============================
>>
>> # Prep output dir
>> mkdir /application/trace/
>> rm -rf /application/trace/*
>>
>> # Create session
>> sudo lttng destroy snapshot-trace-session
>> sudo lttng create snapshot-trace-session --snapshot 
>> --output="/application/trace/"
>> sudo lttng enable-channel --kernel --num-subbuf=8 channelk
>> sudo lttng enable-channel --userspace --num-subbuf=8 channelu
>>
>> # Configure session
>> sudo lttng enable-event --kernel --syscall --all --channel channelk
>> sudo lttng enable-event --kernel --tracepoint "sched*" --channel channelk
>> sudo lttng enable-event --userspace --all --channel channelu
>> sudo lttng add-context -u -t vtid -t procname
>> sudo lttng remove-trigger trig_reboot
>> sudo lttng add-trigger --name=trig_reboot \
>>          --condition=event-rule-matches --type=kernel:syscall:entry \
>>          --name=reboot\
>>          --action=snapshot-session snapshot-trace-session \
>>          --rate-policy=once-after:1
>>
>> # start & list info
>> sudo lttng start
>> sudo lttng list snapshot-trace-session
>> sudo lttng list-triggers
>>
>> #======== test it...
>> sudo reboot
>>
>> #======= reconnect and Nothing :(
>> $ ls -alu /application/trace/
>> drwxr-xr-x    2 u  u       4096 May 15  2024 .
>> drwxr-xr-x   10 u  u       4096 May 15  2024 ..
>>
>>
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