linux-kernel - [PATCH v12 0/3] printk: Make printk() completely async

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Message-Id: <20160513131848.2087-1-sergey.senozhatsky@gmail.com>
Date:	Fri, 13 May 2016 22:18:45 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.com>, Petr Mladek <pmladek@...e.com>,
	Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [PATCH v12 0/3] printk: Make printk() completely async

Hello,

no code changes, just refreshing the series so it'll be easier to
pick up. added Reviwed-by-s and corrected a typo spotted by Petr.

This patch permits to change printk() to operate in completely
asynchronous mode: 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.

against next-20160513


v12:
-- rename printk_kthread_can_run bool flag
-- update printk_kthread_can_run comment (Petr)
-- drop mutex from printk_sync_set(), sysfs writes are synchronised (Petr)

v11:
-- switch default to sync printk
-- make `synchronous' param RW (Andrew, Jan)
-- set RT priority to printk kthread (Andrew)
-- correct comments (Andrew)

v10:
-- simplify printk_kthread_need_flush_console (Jan, Petr)

v9:
-- move need_flush_console assignment down in vprintk_emit (Jan)
-- simplify need_flush_console assignment rules (Petr)
-- clear need_flush_console in printing function (Petr)
-- rename need_flush_console (Petr)

v8:
-- rename kthread printing function (Petr)
-- clear need_flush_console in console_unlock() under logbuf (Petr)

v7:
-- do not set global printk_sync in panic in vrintk_emit() (Petr)
-- simplify vprintk_emit(). drop some of local variables (Petr)
-- move handling of LOGLEVEL_SCHED messages back to printk_deferred()
   so we wake_up_process()/console_trylock() in vprintk_emit() only
   for !in_sched messages

v6:
-- move wake_up_process out of logbuf lock (Jan, Byungchul)
-- do not disable async printk in recursion handling code.
-- rebase against next-20160321 (w/NMI patches)

v5:
-- 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:
-- do not directly wake_up() the printing kthread from vprintk_emit(), need
   to go via IRQ->wake_up() to avoid sched deadlocks (Jan)

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

v2:
- 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

Sergey Senozhatsky (1):
  printk: make printk.synchronous param rw

 Documentation/kernel-parameters.txt |  12 +++
 kernel/printk/printk.c              | 146 +++++++++++++++++++++++++++++++++---
 2 files changed, 148 insertions(+), 10 deletions(-)

-- 
2.8.2.372.g63a3502