[patch RFC 00/29] printk: A new approach - WIP

[Thomas Gleixner]

Folks!

After the recent revert of the threaded printk patches, John and I sat down
and did a thorough analysis of the failure(s). It turned out that there are
several reasons why this failed:

  1) The blurry semantics of console lock which triggers unpleasant
     memories of BKL. That in turn inspired me to flag the new consoles
     with CON_NOBKL and use the nobkl theme throughout the series as I
     could not come up with a better name. :)

  2) The assumption that seperating a printk thread out from console lock,
     but at the same time partially depending on console lock. That's not
     really working out.

  3) The operation of consoles and printk threads was depending solely on
     global state and that state is on/off so it does not really qualify as
     stateful and is therefore not really useful to create a stable
     mechanism.

So I have to correct myself and admit that the revert was the Right Thing
to do. My other statements in that mail [1] still stand.

Nevertheless threaded printing is not only the last missing prerequisite
for enabling RT in mainline, it is also a long standing request from the
enterprise departement.

Synchronous printk is slow especially with serial consoles (which are at
the same time the most reliable ones) where a single character takes ~87
microseconds at 115200 Baud. With enough noise this causes lockup detectors
to trigger and the current "load balancing" approach of handing over the
consoles after one line to an different CPU is just a bandaid which "works"
by chance and makes CPUs spinwait for milliseconds. 

After taking a step back we decided to go for a radically different
approach:

  Create infrastructure which is designed to overcome the current
  console/printk shortcomings and convert drivers one by one.

  A CPU hotplug dejavu. And yes we should have gone for that right away.
  Water down the bridge...

The infrastructure we implemented comes with the following properties:

  - It shares the console list, but only for registration/unregistration
    purposes. Consoles which are utilizing the new infrastructure are
    ignored by the existing mechanisms and vice versa. This allows to
    reuse as much code as possible and preserves the printk semantics
    except for the threaded printing part.

  - The console list walk becomes SRCU protected to avoid any restrictions
    on contexts

  - Consoles become stateful to handle handover and takeover in a graceful
    way.

  - All console state operations rely solely on atomic*_try_cmpxchg() so
    they work in any context.

  - Console locking is per console to allow friendly handover or "safe"
    hostile takeover in emergency/panic situations. Console lock is not
    relevant for consoles converted to the new infrastructure.

  - The core provides interfaces for console drivers to query whether they
    can proceed and to denote 'unsafe' sections in the console state, which
    is unavoidable for some console drivers.

    In fact there is not a single regular (non-early) console driver today
    which is reentrancy safe under all circumstances, which enforces that
    NMI context is excluded from printing directly. TBH, that's a sad state
    of affairs.

    The unsafe state bit allows to make informed decisions in the core
    code, e.g. to postpone printing if there are consoles available which
    are safe to acquire. In case of a hostile takeover the unsafe state bit
    is handed to the atomic write callback so that the console driver can
    act accordingly.
    
  - Printing is delegated to a per console printer thread except for the
    following situations:

       - Early boot
       - Emergency printing (WARN/OOPS)
       - Panic printing

The integration is complete, but without any fancy things, like locking all
consoles when entering a WARN, print and unlock when done. Such things only
make sense once all drivers are converted over because that conflicts with
the way how the existing console lock mechanics work.

For testing we used the most simple driver: a hacked up version of the
early uart8250 console as we wanted to concentrate on validating the core
mechanisms of friendly handover and hostile takeovers instead of dealing
with the horrors of port locks or whatever at the same time. That's the
next challenge. Hack patch will be provided in a reply.

Here is sample output where we let the atomic and thread write functions
prepend each line with the printing context (A=atomic, T=thread):

A[    0.394066] ... fixed-purpose events:   3
A[    0.395130] ... event mask:             000000070000000f

End of early boot, thread starts

TA[    0.396821] rcu: Hierarchical SRCU implementation.

^^ Thread starts printing and immediately raises a warning, so atomic
   context at the emergency priority takes over and continues printing.

   This is a forceful, but safe takeover scenario as the WARN context
   is obviously on the same CPU as the printing thread where friendly
   is not an option.

A[    0.397133] rcu: 	Max phase no-delay instances is 400.
A[    0.397640] ------------[ cut here ]------------
A[    0.398072] WARNING: CPU: 0 PID: 13 at drivers/tty/serial/8250/8250_early.c:123 __early_serial8250_write.isra.0+0x80/0xa0
....
A[    0.440131]  ret_from_fork+0x1f/0x30
A[    0.441133]  </TASK>
A[    0.441867] ---[ end trace 0000000000000000 ]---
T[    0.443493] smp: Bringing up secondary CPUs ...

After the warning the thread continues printing.

....

T[    1.916873] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
T[    1.918719] md: Waiting for all devices to be available before autod

A[    1.918719] md: Waiting for all devices to be available before autodetect

System panics because it can't find a root file system. Panic printing
takes over the console from the printer thread immediately and reprints
the interrupted line.

This case is a friendly handover from the printing thread to the panic
context because the printing thread was not running on the panic CPU, but
handed over gracefully.

A[    1.919942] md: If you don't use raid, use raid=noautodetect
A[    1.921030] md: Autodetecting RAID arrays.
A[    1.921919] md: autorun ...
A[    1.922686] md: ... autorun DONE.
A[    1.923761] /dev/root: Can't open blockdev

So far the implemented state handling machinery holds up on the various
handover and hostile takeover situations we enforced for testing.

Hostile takeover is nevertheless a special case. If the driver is in an
unsafe region that's something which needs to be dealt with per driver.
There is not much the core code can do here except of trying a friendly
handover first and only enforcing it after a timeout or not trying to print
on such consoles.

This needs some thought, but we explicitely did not implement any takeover
policy into the core state handling mechanism as this is really a decision
which needs to be made at the call site. See patch 28.

We are soliciting feedback on that approach and we hope that we can
organize a BOF in Dublin on short notice.

The series is also available from git:

    git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git printk

The series has the following parts:

   Patches  1 - 5:   Cleanups

   Patches  6 - 12:  Locking and list conversion

   Patches 13 - 18:  Improved output buffer handling to prepare for
   	      	     code sharing

   Patches 19 - 29:  New infrastructure implementation

Most of the preparatory patches 1-18 have probably a value on their own.

Don't be scared about the patch stats below. We added kernel doc and
extensive comments to the code:

  kernel/printk/printk_nobkl.c: Code: 668 lines, Comments: 697 lines, Ratio: 1:1.043

Of course the code is trivial and straight forward as any other facility
which has to deal with concurrency and the twist of being safe in any
context. :)

Comments welcome.

Thanks,

	tglx
---
[1] https://lore.kernel.org/lkml/87r11qp63n.ffs@tglx/
---
 arch/parisc/include/asm/pdc.h  |    2 
 arch/parisc/kernel/pdc_cons.c  |   53 -
 arch/parisc/kernel/traps.c     |   17 
 b/kernel/printk/printk_nobkl.c | 1564 +++++++++++++++++++++++++++++++++++++++++
 drivers/tty/serial/kgdboc.c    |    7 
 drivers/tty/tty_io.c           |    6 
 fs/proc/consoles.c             |   12 
 fs/proc/kmsg.c                 |    2 
 include/linux/console.h        |  375 +++++++++
 include/linux/printk.h         |    9 
 include/linux/syslog.h         |    3 
 kernel/debug/kdb/kdb_io.c      |    7 
 kernel/panic.c                 |   12 
 kernel/printk/printk.c         |  485 ++++++++----
 14 files changed, 2304 insertions(+), 250 deletions(-)