[RFC][PATCH v4 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-20160311 with reverted NMI patches.

The patch that makes console_unlock() asynchronous will be posted
later (once this patch set is done; or may be sligthly earlier, so
it'll see wider testing), as a separate further improvement.


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: Skip messages on oops

 Documentation/kernel-parameters.txt |  10 ++
 kernel/printk/printk.c              | 235 +++++++++++++++++++++++++++---------
 2 files changed, 186 insertions(+), 59 deletions(-)

-- 
2.8.0.rc0