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Subject[RFC][PATCHv5 00/13] printk: introduce printing kernel thread


This patch set adds a printk() kernel thread which let us
to print kernel messages to the console from a non-atomic/schedule-able
context, avoiding different sort of lockups, stalls, etc.

Offloading threshold is now in "seconds spent in console_unlock()",
not in "lines printed", this lets us to be a little bit more 'watchdog-aware'
in printk. But we still can (and probably must) do better, tho. May be there
will be a new watchdog API some day, so will be able to make better decisions
in printk.

And a whole bunch of other improvements and tweaks. Like preemption
in console_unlock(), and so on. Any comments and thoughts are appreciated.

-- split some of the patches
-- make offloading time-based (not number of lines printed)
-- move offloading control to per-CPU
-- remove a pessimistic offloading spin from console_unlock()
-- adjust printk_kthread CPU affinity mask
-- disable preemption in console_unlock()
-- always offload printing from user space processes
-- add sync version of emergency_begin API
-- offload from printk_kthread as well, to periodically up() console_sem
-- limit `atomic_print_limit' to `watchdog_thresh'
-- and some other changes...

v3->v4 (Petr, Jan)
-- add syscore notifiers
-- fix 0001 compilation warnings
-- use proper CPU notifiers return values

v2->v3 (Petr, Pavel, Andreas):
-- rework offloading
-- use PM notifiers
-- dropped some patches, etc. etc.

-- introduce printk_emergency mode and API to switch it on/off
-- move printk_pending out of per-CPU memory
-- add printk emergency_mode sysfs node
-- switch sysrq handlers (some of them) to printk_emergency
-- cleanus/etc.

Sergey Senozhatsky (13):
printk: move printk_pending out of per-cpu
printk: introduce printing kernel thread
printk: add sync printk_emergency API
printk: add enforce_emergency parameter
printk: enable printk offloading
printk: register PM notifier
printk: register syscore notifier
printk: set watchdog_thresh as maximum value for atomic_print_limit
printk: add auto-emergency enforcement mechanism
printk: force printk_kthread to offload printing
printk: always offload printing from user-space processes
printk: do not cond_resched() when we can offload
printk: move offloading logic to per-cpu

Documentation/admin-guide/kernel-parameters.txt | 17 +
include/linux/console.h | 5 +
kernel/printk/printk.c | 435 ++++++++++++++++++++++--
3 files changed, 433 insertions(+), 24 deletions(-)


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