C855B
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Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 5:22 pm

One of our 400s "froze" this morning in waking up from sleep. (I'm presuming it goes to sleep, or whatever it does when it idles the video.) The screen stayed blank, although it was active. I quickly jumped in given this particular operator's penchant for pounding keys or - heaven forbid - yanking the plug out of the wall. I had to gently remind her to never, ever, ever "just unplug it", like she did with the Mac she was used to.

In my hands it wasn't a total freeze-up, I could tell that some process was hogging >100% of the CPU, including that the green LED was on solid and never blinked with "disk" activity. I tried various Alt-Ctrl-[F-key] combos, one of which got me to a command-line login prompt after about five minutes, so I don't know which one actually worked. Adding insult, I could start the login, but it took so long to get to the password line I'd get the 60-second timeout warning, and would have to wait another 3-4 minutes for the login prompt to try again... which was never successful due to the timeout.

I pulled the plug. System is running fine, so (apparently) I didn't corrupt the file system.

So... both Macs and PCs have some version of a "three-finger salute" key combo which forces a hard reboot. Does the 400 have an equivalent? The problem with the Alt-Ctrl-[F-key] interrupt to get to the command line is that it is apparently of no use if there's some low-level process running away and pounding on the processor.

GlowInTheDark
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 6:10 pm

So... both Macs and PCs have some version of a "three-finger salute" key combo which forces a hard reboot. Does the 400 have an equivalent? The problem with the Alt-Ctrl-[F-key] interrupt to get to the command line is that it is apparently of no use if there's some low-level process running away and pounding on the processor.
Several comments:

1) Modern version of Windows don't really have Ctrl/Alt/Del like DOS (and DOS-ish versions of WIndows) did. It is entirely in software now - so if the machine is really hung, C/A/D isn't going to help. (I'm not here to debate Windows trivia; you know what I mean and if you don't, post it somewhere else).

2) Does the Mac have something like that? I have a Mac but I never knew that.

3) Pre-systemd, Raspbian did have C/A/D wired to do "something" - that something was entirely software driven, controlled by the /etc/inittab file. I know that I routinely changed it when I setup a new machine, because I didn't like whatever the default was. In any case, as with Windows (see #1 above), it was entirely software, so not much use if the machine was actually hung.

I think that functionality is now gone with systemd. Not sure (since I haven't tried in a long time) what (if anything) C/A/D does on a current Raspbian (with systemd).

4) Don't be afraid to pull the plug. Sometimes ya gotta. The point is: It isn't clear to me when a "three finger salute" as you call it would help. I mean, either it's hung or it isn't. Every so often, I have to hard crash (i.e., power cycle) one of my Pis; I always hold my breath on the reboot to make sure nothing bad has happened. So far, I've never lost anything.

5) There is something that has something to do with hitting Sys-Req and then REISUB, that I'm sure other posters will tell you about any second now, but it has never worked for me. Again, either it's hung or it isn't.

6) Doing Ctrl/Alt/F2 to get to a command line and then logging in there (or, equivalently, ssh'ing in) is about the best you can do. Sometimes it saves your bacon; most of the time, not.
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KeithMck
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 6:12 pm

As far as I'm aware, CTRL+ALT+DEL works as with any other Linux O/S - & there is that F10 key that should also shut it down.

bls
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 6:15 pm

I haven't investigated, but systemd does have a ctrl-alt-del.target that appears to do a reboot.
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C855B
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 6:38 pm

Thanks for the quick come-back, guys.
GlowInTheDark wrote:
Tue May 18, 2021 6:10 pm
2) Does the Mac have something like that? I have a Mac but I never knew that.
Command-Control-Eject (or C-C-Power on recent laptops).
4) Don't be afraid to pull the plug. Sometimes ya gotta. ... So far, I've never lost anything.
That's been my experience, too, but on Pis where I could rebuild everything in about an hour if I had to. This particular workstation had stuff on it that would take a couple of days to restore. (Yeah, image backup. The roundtuit just moved up the task list.) The hesitancy to yank power is from the general RPi advice added to decades of experience on "lesser" systems where accidentally halting the processor in the midst of a directory table write was bad bad bad.
KeithMck wrote: As far as I'm aware, CTRL+ALT+DEL works as with any other Linux O/S - & there is that F10 key that should also shut it down.
bls wrote: I haven't investigated, but systemd does have a ctrl-alt-del.target that appears to do a reboot.
I'll keep trying that. Since it's a systemd parm it must be a software function. Since the freeze was something hammering the processor really hard, it might take holding the keys down for a lot more than a couple of seconds as we wait for the interrupt manager to get around to servicing the keyboard.

Thanks again.

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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 6:49 pm

C855B wrote:
Tue May 18, 2021 6:38 pm
bls wrote: I haven't investigated, but systemd does have a ctrl-alt-del.target that appears to do a reboot.
I'll keep trying that. Since it's a systemd parm it must be a software function. Since the freeze was something hammering the processor really hard, it might take holding the keys down for a lot more than a couple of seconds as we wait for the interrupt manager to get around to servicing the keyboard.

Thanks again.
I neglected to mention that (at least on my RasPiOS Lite system) this target is disabled. Maybe it's enabled on RasPiOS with Desktop, I can't easily check. There seem to be quite a few search hits about Disabling it, but I didn't see any about enabling it on my quick search.
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Tue May 18, 2021 10:44 pm

GlowInTheDark wrote:
Tue May 18, 2021 6:10 pm
5) There is something that has something to do with hitting Sys-Req and then REISUB
Magic SysRq can be very picky about how it activates, but I've got it to work on a seemingly hung Raspberry Pi. It won't work if the kernel has done a panic
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 5:18 am

https://www.kernel.org/doc/html/latest/ ... sysrq.html

Also: https://raspberrypi.stackexchange.com/q ... -a-monitor
Raspberry Pi 4 with Raspberry Pi OS:

The SysReq key, by default, seems to be Right Alt + PrintScreen held down together.

The graceful shutdown key sequence is "REISUO".

So, all together:

Keep holding down Right Alt + PrintScreen, and with your other hand, press the sequence keys, leaving one second wait time between each keystroke: R + E + I + S + U + O
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 5:28 am

I've noticed these exact symptoms myself after the screen saver utility is left unattended for a while.
Rather annoying as pulling the plug was the only viable solution.

I simply disabled the screen saver utility and have never again had this issue ever.
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GlowInTheDark
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 6:36 am

The graceful shutdown key sequence is "REISUO".
The usual final letter is B not O. B means reboot (which is what this thread is about). O would be to just shutdown (without rebooting).
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GlowInTheDark
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 6:38 am

I've noticed these exact symptoms myself after the screen saver utility is left unattended for a while.
Is this unique to the Pi400? I've never seen it on normal Pis (and I use xscreensaver on all of them). I do not own and have never used a 400.
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 7:22 am

GlowInTheDark wrote:
Wed May 19, 2021 6:38 am
I've noticed these exact symptoms myself after the screen saver utility is left unattended for a while.
Is this unique to the Pi400? I've never seen it on normal Pis (and I use xscreensaver on all of them). I do not own and have never used a 400.
I don't own a pi400 either, but I've seen it on several pi4b, a pi3A+, a pi3b and three pi zero/w leading me to believe it is not specific to one model of Pi, but the screen saver utility itself.

By "unattended for a while", I mean multiple hours, but I have also noticed it after only half an hour or so as well.
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GlowInTheDark
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Wed May 19, 2021 7:32 am

I don't own a pi400 either, but I've seen it on several pi4b, a pi3A+, a pi3b and three pi zero/w leading me to believe it is not specific to one model of Pi, but the screen saver utility itself.
Interesting.
By "unattended for a while", I mean multiple hours, but I have also noticed it after only half an hour or so as well.
My machines are on 24/7, so are often running the screensaver for a day or more.
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Thu May 20, 2021 8:49 am

GlowInTheDark wrote:
Wed May 19, 2021 6:38 am
Is this unique to the Pi400? I've never seen it on normal Pis (and I use xscreensaver on all of them). I do not own and have never used a 400.
Haven't seen it on Pi and Pi2 machines. The 400 freezes after been unattended for some (usually many) hours. c-a-d doesn't work anymore when that happens. Thought it was just me.

But: It freezes on wake-up from the screensaver, so I see the desktop. Always. Might be something unrelated, who knows. One of those things no one will ever find out.
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Piromancer
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Sat Jun 05, 2021 1:25 pm

I too encountered this a few times on my Pi 400, which would not display anything after being left unattended for 3-4 hours. The Pi was still working, as I could see the SSD light blink when I did a Ctrl-Alt-T to bring up a terminal window or do a Shift-Ctrl-Esc to bring up the Task Manager (no display mind you). I would do a Ctrl-Alt-Del, then Enter to shutdown.

What worked for me was to disable Screen Blanking in Raspberry Pi Configuration | Display tab. Even with this setting disabled, my 4K monitor is still able to go into Standby mode after idling for a while, so still saving power and LED back-light lifespan.

I don't know if this tip is related to what you are facing, but it did seem to work for my Pi 400.
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KeithMck
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Re: Is There a Forced Reboot? A "Three-Finger Salute" Equivalent?

Sun Jun 06, 2021 9:10 am

Check your screensaver & power settings, it could be that you have it set to turn the monitor off after a certain amount of inactivity.

(Sometimes, if a monitor has been blanked, it will turn off the power after (often) 20mins.)

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