[RFC][PATCH v8 0/2] printk: Make printk() completely async

From: Sergey Senozhatsky
Date: Thu Mar 24 2016 - 11:45:50 EST


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.

against next-20160324

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

-- 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

-- 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)

-- 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

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

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

- use dedicated printk workqueue with WQ_MEM_RECLAIM bit
- fallback to system-wide workqueue only if allocation of printk_wq has
- 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 | 104 ++++++++++++++++++++++++++++++++----
2 files changed, 104 insertions(+), 10 deletions(-)