[RFC][PATCH v2 0/2] printk: Make printk() completely async

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

Hello,

 RFC

 The patch set is a slightly updated Jan Kara's patches. The majority
of the changes are in 0001 (described below in details).

I've performed some tests and haven't spotted any problems, I'll
certainly do more tests. Any feedback is appreciated.

I had additional patches on top of the series, but I decided not to post
them. I'm unhappy with console semaphore, this must be a separate mail
loop and a patch set, some day.


The patch set is against next-20160304 with reverted NMI patches.


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              | 230 +++++++++++++++++++++++++++---------
 2 files changed, 181 insertions(+), 59 deletions(-)

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2.8.0.rc0