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Subject[PATCHSET] printk, netconsole: implement reliable netconsole
In a lot of configurations, netconsole is a useful way to collect
system logs; however, all netconsole does is simply emitting UDP
packets for the raw messages and there's no way for the receiver to
find out whether the packets were lost and/or reordered in flight.

printk already keeps log metadata which contains enough information to
make netconsole reliable. This patchset does the followings.

* Make printk metadata available to console drivers. A console driver
can request this mode by setting CON_EXTENDED. The metadata is
emitted in the same format as /dev/kmsg. This also makes all
logging metadata including facility, loglevel and dictionary
available to console receivers.

* Implement extended mode support in netconsole. When enabled,
netconsole transmits messages with extended header which is enough
for the receiver to detect missing messages.

* Implement netconsole retransmission support. Matching rx socket on
the source port is automatically created for extended targets and
the log receiver can request retransmission by sending reponse
packets. This is completely decoupled from the main write path and
doesn't make netconsole less robust when things start go south.

* Implement netconsole ack support. The response packet can
optionally contain ack which enables emergency transmission timer.
If acked sequence lags the current sequence for over 10s, netconsole
repeatedly re-sends unacked messages with increasing interval. This
ensures that the receiver has the latest messages and also that all
messages are transferred even while the kernel is failing as long as
timer and netpoll are operational. This too is completely decoupled
from the main write path and doesn't make netconsole less robust.

* Implement the receiver library and simple receiver using it
respectively in tools/lib/netconsole/libncrx.a and tools/ncrx/ncrx.
In a simulated test with heavy packet loss (50%), ncrx logs all
messages reliably and handle exceptional conditions including
reboots as expected.

An obvious alternative for reliable loggin would be using a separate
TCP connection in addition to the UDP packets; however, I decided for
UDP based retransmission and ack mechanism for the following reasons.

* kernel side doesn't get simpler by using TCP. It'd still need to
transmit extended format messages, which BTW are useful regardless
of reliable transmission, to match up UDP and TCP messages and
detect missing ones from TCP send buffer filling up. Also, the
timeout and emergency transmission support would still be necessary
to ensure that messages are transmitted in case of, e.g., network
stack faiure. It'd at least be about the same amount of code as the
UDP based implementation.

* Receiver side might be a bit simpler but not by much. It'd still
need to keep track of the UDP based messages and then match them up
with TCP messages and put messages from both sources in order (each
stream may miss different ones) and would have to deal with
reestablishing connections after reboots. The only part which can
completely go away would be the actual ack and retransmission part
and that isn't a lot of logic.

* When the network condition is good, the only thing the UDP based
implementation adds is occassional ack messages. TCP based
implementation would end up transmitting all messages twice which
still isn't much but kinda silly given that using TCP doesn't lower
the complexity in meaningful ways.

This patchset contains the following 16 patches.


0001-0005 implement extended console support in printk.

0006-0011 are prep patches for netconsole.

0012-0014 implement extended mode, retransmission and ack support.

0015 implements receiver library, libncrx, and a simple receiver using
the library, ncrx.

0016 updates documentation.

As the patchset touches both printk and netconsole, I'm not sure how
these patches should be routed once acked. Either -mm or net should
work, I think.

This patchset is on top of linus#master[1] and available in the
following git branch.

git:// review-netconsole-ext

diffstat follows. Thanks.

Documentation/networking/netconsole.txt | 95 +++
drivers/net/netconsole.c | 800 +++++++++++++++++++++++-----
include/linux/console.h | 1
include/linux/printk.h | 16
kernel/printk/printk.c | 411 +++++++++++---
tools/Makefile | 16
tools/lib/netconsole/Makefile | 36 +
tools/lib/netconsole/ncrx.c | 906 ++++++++++++++++++++++++++++++++
tools/lib/netconsole/ncrx.h | 204 +++++++
tools/ncrx/Makefile | 14
tools/ncrx/ncrx.c | 143 +++++
11 files changed, 2419 insertions(+), 223 deletions(-)


[1] 497a5df7bf6f ("Merge tag 'stable/for-linus-4.1-rc0-tag' of git://")

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