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nerves_heart

This is a replacement for Erlang's heart port process specifically for Nerves-based devices. It is installed by default on all Nerves devices.

Heart monitors the Erlang runtime using heart beat messages. If the Erlang runtime becomes unresponsive, heart reboots the system. This implementation of heart is fully compatible with the default implementation and provides the following changes:

1. Supports a hardware watchdog timer (WDT) so that if both the Erlang runtime and heart become unresponsive, the hardware watchdog can reboot the system
2. Simplifies the code base. Code not needed for Nerves devices has been removed to make the program easier to audit.
3. Provides diagnostic information. See Nerves.Runtime.Heart for an easier interface to get this.
4. Directly calls sync(2) and reboot(2) when Erlang is unresponsive. The reboot call is not configurable nor is it necessary to invoke another program.
5. Trigger watchdog-protected reboots and shutdowns
6. Support for an application-level initialization handshake. This lets your application guard against conditions that cause Erlang and Nerves Heart to think that everything is ok when it's not. See discussion below.
7. Support for snoozing timeouts and assuring an uninterrupted amount of time at start. This makes heart friendlier to remote debug where it's nice when the watchdog doesn't get in your way of debugging. These timeouts are limited to avoid accidentally putting the device in a state where it can't recover.
8. Includes many regression tests make it easier to modify and maintain.

Timeouts and semantics

The following timeouts are important to nerves_heart:

• Erlang VM heart beat timeout - the time interval for Erlang heart beat messages
• Watchdog Timer (WDT) timeout - the time interval that before the WDT resets the device
• WDT pet timeout - the time interval that nerves_heart pets the WDT

The WDT pet timeout is always shorter than the WDT timeout. The Erlang VM heart beat timer may be longer or shorter than the WDT timeout. Heart beat messages from Erlang reset the heart beat timer and pet the WDT. In the case that Erlang sends heart beat messages frequent enough to satisfy the heart beat timeout, but at a shorter interval than the WDT pet timeout, nerves_heart will automatically pet the WDT.

In the event that Erlang is unresponsive and if nerves_heart is still able to run, it will reboot the system by calling sync(2) and then reboot(2). If even nerves_heart is unresponsive, the WDT will reset the system. This will also happen if sync(2) or reboot(2) hang nerves_heart.

If you stop the Erlang VM gracefully (such as by calling :init.stop/0), Erlang will tell nerves_heart to exit without rebooting. This, however, will stop petting the WDT which will also lead to a system reset if the Linux kernel has CONFIG_WATCHDOG_NOWAYOUT=y in its configuration. All official Nerves systems specify this option.

WDT pet decisions made by nerves_heart:

1. Pet the WDT on start. This is important since it's unknown how much time has passed since the WDT has been started and when nerves_heart is started. This prevents a reboot due to a slow boot time.
2. Pet the WDT on Erlang heart beat messages AND pet timeout events. This makes the Erlang VM heart beat timeout the critical one in deciding when reboots happen.
3. Pet the WDT once on graceful shutdown and requested crash dumps. The intention is to give other shutdown code or crash dump preparation code the full WDT timeout interval.

Using

If nerves_heart is part of your Nerves system, then all that you need to do is enable it in your rel/vm.args file by adding the line:

-heart

If nerves_heart is not part of your system, you'll need to compile it and copy it over the Erlang-provided heart program in /usr/lib/erlang/erts-x.y/bin.

Once you have heart running, if you have a hardware watchdog on your board, you'll see a message printed to the console on your device with the pet interval:

heart: Kernel watchdog activated (interval 5s)

The interval that Erlang needs to communicate with heart can be different from the interval that heart pets the hardware watchdog. The default for Erlang is 60 seconds. If this is too long, update your rel/vm.args like follows:

-heart
-env HEART_BEAT_TIMEOUT 30

The heart beat timeout has to be greater than 10 seconds per the Erlang documentation.

If you need to change the watchdog path, you can do this through an environment variable.

-env HEART_WATCHDOG_PATH /dev/watchdog1

The following table shows the environment variables that affect Nerves Heart:

Linux kernel configuration

All official Nerves systems have Linux configured of Nerves Heart.

Nerves Heart expects the kernel watchdog device driver to be enabled with the no way out option:

CONFIG_WATCHDOG=y
CONFIG_WATCHDOG_NOWAYOUT=y

Linux provides several hardware watchdog driver so select the option that matches your device. You may need to add a section to your device tree to configure the driver. For example, if using an external watchdog and petting it via a GPIO, you will need to specify which GPIO in the device tree.

Application-level initialization handshake

Erlang's heart module has a very useful feature of letting you specify a callback to extend the heart beat protection beyond the VM. See :heart.set_callback/2. To use it, you specify a callback function that returns :ok if your application seems is ok. If it's not ok, then Erlang won't send a heart beat message to the heart process and the device will eventually reboot.

The problem with this mechanism is that if something causes the code that registers the callback to not be run, the device could continue to run without knowing that its in a bad state. As an aside, a non-Nerves way of handling this would be to exit the VM with an error message. On devices, running in a degraded state can be useful to keep important services going and allowing time for remote troubleshooting.

Here's the flow for using the application-level initialization handshake:

1. Set the HEART_INIT_TIMEOUT environment variable to the number of seconds to wait for the initializing handshake.
2. Call :heart.set_callback/2 in your program
3. Call Nerves.Runtime.Heart.init_complete/0 or :heart.setcmd('init_done'). If you call the latter, be sure to do it in a different process than the your heart callback uses or you'll get a hang.

To set HEART_INIT_TIMEOUT, edit the rel/vm.args.eex and update the heart section to look like this:

## Enable heartbeat monitoring of the Erlang runtime system
-heart
-env HEART_BEAT_TIMEOUT 30

## Require an initialization handshake within 15 minutes
-env HEART_INIT_TIMEOUT 900

With this configuration, Nerves Heart will operate as normal with petting the hardware watchdog and expecting Erlang heart beat messages up. If init_done is passed, Nerves Heart will continue working as normal. However, if init_done is not sent at the 15 minute mark, Nerves Heart will pet the hardware WDT one last time and exit. This will cause the Erlang VM to exit and if the VM decides to not be happy about this, the hardware WDT will reset.

Runtime diagnostic information

Both nerves_heart and the Linux watchdog subsystem can return useful information via :heart.get_cmd/0. According to the Erlang/OTP documentation, this function really should return the temporary reboot command, but it's commented as unsupported in the Erlang version of heart. Given that there isn't a better function call for returning this information, this one is used.

Here's an example run:

iex> :heart.get_cmd
{:ok,
 'program_name=nerves_heart\nprogram_version=2.0.0\nheartbeat_timeout=60\nheartbeat_time_left=53\nwdt_pet_time_left=103\nwdt_identity=OMAP Watchdog\nwdt_firmware_version=0\nwdt_options=settimeout,magicclose,keepaliveping,\nwdt_time_left=116\nwdt_pre_timeout=0\nwdt_timeout=120\nwdt_last_boot=power_on\n'}

The format is "key=value\n". The keys are either from nerves_heart or from Linux watchdog ioctl functions.

If you're using Nerves, Nerves.Runtime.Heart has a friendlier interface:

iex> Nerves.Runtime.Heart.status!
%{
  program_name: "nerves_heart",
  program_version: %Version{major: 2, minor: 0, patch: 0},
  heartbeat_timeout: 60,
  heartbeat_time_left: 53,
  init_handshake_happened: true,
  init_handshake_timeout: 900,
  init_handshake_time_left: 0,
  wdt_identity: "OMAP Watchdog",
  wdt_firmware_version: 0,
  wdt_last_boot: :power_on,
  wdt_options: [:settimeout, :magicclose, :keepaliveping],
  wdt_pre_timeout: 0,
  wdt_time_left: 116,
  wdt_pet_time_left: 103,
  wdt_timeout: 120
}

The following table describes keys and their values:

Reboot and power off

The :heart.set_cmd/1 function can be used to trigger reboots and shutdowns.

To understand why, it's good to review the alternatives. The other ways of rebooting with Nerves are either by exiting the Erlang VM via :init.stop/0 or by running the reboot(8) or poweroff(8) shell commands. The latter two work by sending a signal to PID 1 (erlinit) that kills all OS processes and does the reboot and power off. These work well in practice, but have had failures blamed on them that can be further reduced by using Nerves Heart. For example, one failure was found where OS processes couldn't be started and therefore reboot(8) wasn't being called. This ended up causing a hang that wasn't resolved for a long time. Since using Nerves Heart to reboot does not create OS processes, it would not have this issue and if communication problems between Nerves Heart and Erlang arise, that will be detected by the heart beat timers. There's lots to say about this topic. Suffice it to say that the heart process has advantages in simplicity and control of the hardware WDT to make this process more reliable.

To use, run either:

iex> :heart.set_cmd("guarded_reboot")

or

iex> :heart.set_cmd("guarded_poweroff")

IMPORTANT: nerves_runtime v0.13.2 and later makes these calls for you when running Nerves.Runtime.reboot/0 and Nerves.Runtime.poweroff/0 if it detects that this feature is supported.

Nerves Heart will notify PID 1 of the intention to reboot or poweroff. However, the WDT will be pet for the last time. You should then call either :init.stop/0 (graceful) or :erlang.halt/0 (ungraceful) to exit the Erlang VM. Both erlinit and the WDT will prevent shutdown from not completing.

It's also possible to do a totally ungraceful reboot or shutdown if you need a very immediate response. The "guarded_immediate_reboot" and "guarded_immediate_poweroff" commands do this.

Testing

It's reassuring to know that heart does what it's supposed to do since it should only rarely take action. If you'd like to verify that a hardware watchdog works, you can instruct heart to stop petting the hardware watchdog by running:

iex> :heart.set_cmd("disable_hw")

If you'd like to verify how the Erlang VM handles the heart process detecting an unresponsive VM, you can instruct heart to stop as it would if it timed out on the VM by running:

iex> :heart.set_cmd("disable_vm")

Snoozing

If you're debugging a watchdog or Erlang heart issue, it can be really helpful to disable reboots temporarily. This is called snoozing and it works by telling heart to pet the hardware watchdog and keep the Erlang VM running even if it should reboot. This is effective for the next 15 minutes every time you run it.

iex> :heart.set_cmd("snooze")

If :heart.set_cmd/1 doesn't return, that means that the Erlang heart process is stuck. This is probably due to the health check callback (registered by :heart.set_callback/2) taking a long time. To get past this, send the USR1 signal to Nerves Heart like this:

iex> :os.cmd('killall -USR1 heart')

The reason snoozing forever isn't supported is to avoid keeping a device on forever in a bad state. For example, it would be unfortunate to start a debug session on a remote device and accidentally mess it up in a way where you lose connectivity until the next reboot.

Debugging

Nerves Heart writes to the kernel's logger to aid debug if something unexpected happens. Only errors are logged by default. If you'd like informational messages as well, set the HEART_VERBOSE environment variable in your vm.args.eex:

-env HEART_VERBOSE 2

If you want Nerves Heart to be completely stealth in its prints for some reason, set HEART_VERBOSE to 0:

-env HEART_VERBOSE 0

Heart set_cmd summary

The following commands can be sent to Nerves Heart via :heart.set_cmd:

License

This production code in this project is Erlang/OTP's heart.c with custom modifications for use with Nerves. This file is licensed under Apache-2.0. Without additional modification, no other license is used.

This project does contain source with other licenses including GPL-2.0-only. Files containing these licenses are clearly marked as required by the REUSE recommendations.

Exceptions to Apache-2.0 licensing are:

• Configuration and data files are licensed under CC0-1.0
• Documentation is CC-BY-4.0
• Linux kernel headers for MacOS development are GPL-2.0-only with the Linux syscall note.