Re: printk meeting at LPC

[John Ogness]

On 2019-09-18, Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx> wrote:
>>>> 2. A kernel thread will be created for each registered console,
>>>> each responsible for being the sole printers to their respective
>>>> consoles. With this, console printing is _fully_ decoupled from
>>>> printk() callers.
>>> 
>>> sysrq over serial?
>>> 
>>> offloading this to kthread may be unreliable.
>> 
>> But we also talked about an "emergency flush" which will not wait for
>> the kthreads to finish and just output everything it can find in the
>> printk buffers (expecting that the consoles have an "emergency"
>> handler. We can add a sysrq to do an emergency flush.

The problem with only a flush here is that the sysrq output may not fit
in the ringbuffer (ftrace, for example). It probably makes more sense to
have a switch to enter/exit "synchronous state", where all atomic
consoles are flushed upon enter and all future printk's are synchronous
on atomic consoles until exit.

I expect sysrq to be the only valid use of "synchronous state" other
than oops/panic. Although I suppose PeterZ would like a boot argument to
always run the consoles in this state.

> I agree that when consoles have ->atomic_write then it surely makes
> sense to switch to emergency mode. I like the emergency state
> approach, but I'm not sure how it can be completely invisible to the
> rest of the system.  Quoting John:
>
>     : Unlike oops_in_progress, this state will not be visible to
>     : anything outside of the printk infrastructure.
>
> For instance, tty/sysrq must be able to switch printk emergency
> on/off.

The switch/flush _will_ be visible. But not the state. So, for example,
it won't be possible for some random driver to determine if we are in an
emergency state. (Well, I don't know if oops_in_progress will really
disappear. But at least the printk/console stuff will no longer rely on
it.)

> We also figured out that some PM (hibernation/suspend/etc.) stages
> (very early and/or very late ones) [2] also should have printk in
> emergency mode, plus some other parts of the kernel [3].
>
> [1] https://lore.kernel.org/lkml/20170815025625.1977-4-sergey.senozhatsky@xxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20170815025625.1977-7-sergey.senozhatsky@xxxxxxxxx/
> [3] https://lore.kernel.org/lkml/20170815025625.1977-8-sergey.senozhatsky@xxxxxxxxx/

Thanks for bringing up that RFC thread again. I haven't looked at it in
over a year. I will go through it again to see if there is anything I've
overlooked. Particularly the suspend stuff.

John Ogness