[lttng-dev] Capturing snapshot on kernel panic

Thu May 16 14:22:52 EDT 2024

Thanks Kienan for these quick suggestions,
we'll investigate the pmem route (I was not aware of the lttng-cash
utility, it's pretty nice) even if I'm not sure how fast it would burn
through our SSD, it might still be worth trying.
As for kexec-tool, it's not officially supported on our embedded modules
unfortunately, so we might be SOL there. We may have to try to add our own
trace-point in kernel to use as trigger.
Cheers
Damien

On Thu, May 16, 2024 at 8:12 AM Kienan Stewart <kstewart at efficios.com>
wrote:

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

-- 
*Damien Berget*
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.lttng.org/pipermail/lttng-dev/attachments/20240516/4c7dfafc/attachment.htm>