Re: printk meeting at LPC

[Sergey Senozhatsky]

On (09/17/19 22:08), Steven Rostedt wrote:
> > On (09/13/19 15:26), John Ogness 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?
> > 
> > What we currently have is hacky, but, as usual, is a "best effort":
> > 
> > 	>> serial driver IRQ  
> > 
> > 	serial_handle_irq()		[console driver]
> > 	 uart_handle_sysrq_char()
> > 	  handle_sysrq()
> > 	   printk()
> > 	    call_console_drivers()
> > 	     serial_write()		[re-enter console driver]
> > 
> > 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.

I'm sorry, I wasn't there, so I'm surely is missing on some details.

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.
That already means that printk emergency knob should be visible to the
rest of the kernel. A long time ago, we had printk_emergency_begin_sync()
and printk_emergency_end_sync(), which would define reentrable
printk_emergency blocks [1]:

	printk_emergency_begin_sync();
	handle_sysrq();
	printk_emergency_end_sync();

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/

	-ss