[RFC][PATCH v5 0/2] printk: Make printk() completely async

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Hello,

 RFC

 The patch set is based on slightly updated Jan Kara's patches.

This patch set makes printk() completely asynchronous: new messages
are getting upended to the kernel printk buffer, but instead of 'direct'
printing the actual print job is performed by a dedicated kthread.
This has the advantage that printing always happens from a schedulable
context and thus we don't lockup any particular CPU or even interrupts.

The patch set is against next-20160318 with reverted NMI patches.

the series in total has 3 patches:
- printk: Make printk() completely async
- printk: Make wake_up_klogd_work_func() async
- printk: make console_unlock() async

per discussion, "printk: make console_unlock() async" will be posted
later on.

v5 (Sergey):
-- make printk.synchronous RO (Petr)
-- make printing_func() correct and do not use wait_queue (Petr)
-- do not panic() when can't allocate printing thread (Petr)
-- do not wake_up_process() only in IRQ, prefer vprintk_emit() (Jan)
-- move wake_up_klogd_work_func() to a separate patch (Petr)
-- move wake_up_process() under logbuf lock so printk recursion logic can
   help us out
-- switch to sync_print mode if printk recursion occured
-- drop "printk: Skip messages on oops" patch

v4 (Sergey):
-- do not directly wake_up() the printing kthread from vprintk_emit(), need
   to go via IRQ->wake_up() to avoid sched deadlocks (Jan)

v3 (Sergey):
-- use a dedicated kthread for printing instead of using wq (Jan, Tetsuo, Tejun)

v2 (Sergey):
- use dedicated printk workqueue with WQ_MEM_RECLAIM bit
- fallback to system-wide workqueue only if allocation of printk_wq has
  failed
- do not use system_wq as a fallback wq. both console_lock() and onsole_unlock()
  can spend a significant amount of time; so we need to use system_long_wq.
- rework sync/!sync detection logic
  a) we can have deferred (in_sched) messages before we allocate printk_wq,
     so the only way to handle those messages is via IRQ context
  b) even in printk.synchronous mode, deferred messages must not be printed
     directly, and should go via IRQ context
  c) even if we allocated printk_wq and have !sync_printk mode, we must route
     deferred messages via IRQ context
- so this adds additional bool flags to vprint_emit() and introduces a new
  pending bit to `printk_pending'
- fix build on !PRINTK configs


Jan Kara (2):
  printk: Make printk() completely async
  printk: Make wake_up_klogd_work_func() async

 Documentation/kernel-parameters.txt |  10 ++
 kernel/printk/printk.c              | 222 ++++++++++++++++++++++++++----------
 2 files changed, 174 insertions(+), 58 deletions(-)

-- 
2.8.0.rc0