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.senozhat...@gmail.com/ [2] https://lore.kernel.org/lkml/20170815025625.1977-7-sergey.senozhat...@gmail.com/ [3] https://lore.kernel.org/lkml/20170815025625.1977-8-sergey.senozhat...@gmail.com/ -ss