[PATCH printk v3 00/15] printk/for-next

John Ogness
2022-04-19T23:46:47Z

This is v3 of a series to implement a kthread for each registered
console. v2 is here [0]. The kthreads locklessly retrieve the
records from the printk ringbuffer and also do not cause any lock
contention between each other. This allows consoles to run at full
speed. For example, a netconsole is able to dump records much
faster than a serial or vt console. Also, during normal operation,
printk() callers are completely decoupled from console printing.

There are situations where kthread printing is not sufficient. For
example, during panic situations, where the kthreads may not get a
chance to schedule. In such cases, the current method of attempting
to print directly within the printk() caller context is used. New
functions printk_prefer_direct_enter() and
printk_prefer_direct_exit() are made available to mark areas of the
kernel where direct printing is preferred. (These should only be
areas that do not occur during normal operation.)

This series also introduces pr_flush(): a might_sleep() function
that will block until all active printing threads have caught up
to the latest record at the time of the pr_flush() call. This
function is useful, for example, to wait until pending records
are flushed to consoles before suspending.

Note that this series does *not* increase the reliability of console
printing. Rather it focuses on the non-interference aspect of
printk() by decoupling printk() callers from printing (during normal
operation). Nonetheless, the reliability aspect should not worsen
due to this series.

John Ogness

[0] https://lore.kernel.org/lkml/20220405132535.649171-1-john.ogness@linutronix.de

Changes since v2:

- Threaded printers no longer care about preferred direct printing.
As with v1, they will print whenever they are not blocked.

- Provide a separate patch to fix a missing memory barrier in
wake_up_klogd() and add memory barrier comments to all
appropriate @log_wait usage sites.

- Provide a separate patch to wake all waiters.

- Provide a separate patch to wake waiters for deferred console
output and add comments explaining why.

- Introduce console_lock_single_hold() and
console_unlock_single_release() to acquire @console_sem and lock a
single threaded printer. This allows console start/stop and
console unregistration with synchronized con->flags and without
disturbing other threaded printers.

- Introduce __console_is_usable() to avoid some redundance between
threaded and direct printing code.

- Do not create a printer thread if con->write() is not set. (I do
not understand why we even allow registration if con->write() is
not set. The checks were added in 2.1.31 for no obvious reason.)

- Only allow handovers between console_trylock() contexts. A
console_lock() context cannot handover the console_lock to a
console_trylock() context because the blocked kthreads would need
to be unblocked via mutex.

- console_flush_all() returns true only if at least one console is
usable and all messages to all usable consoles were printed.
Otherwise it returns false.

- Remove redundant panic check in console_unlock().

- Rename printk_console_msg() to con_printk() and use syntax similar
to dev_printk(). (I did not name it console_printk() because there
already exists a symbol with that name.)

- Remove blocked check in register_console() since it is always
true.

- In unregister_console(), stop the kthread after the console has
been removed from the list. Use the per-console mutex for
synchronized kthread stopping.

- Use the console_lock for synchronized activation of the fallback
permanent direct printing mode.

- Use the same checks in printer_should_wake() as in
printk_kthread_func() to avoid infinite loop danger.

- Rename PRINTK_PENDING_OUTPUT flag to PRINTK_PENDING_DIRECT_OUTPUT.

- Expand commit messages relating to memory barriers, kthreads, and
the usage of the per-console mutex.

John Ogness (15):
printk: rename cpulock functions
printk: cpu sync always disable interrupts
printk: add missing memory barrier to wake_up_klogd()
printk: wake up all waiters
printk: wake waiters for safe and NMI contexts
printk: get caller_id/timestamp after migration disable
printk: call boot_delay_msec() in printk_delay()
printk: add con_printk() macro for console details
printk: refactor and rework printing logic
printk: move buffer definitions into console_emit_next_record() caller
printk: add pr_flush()
printk: add functions to prefer direct printing
printk: add kthread console printers
printk: extend console_lock for proper kthread support
printk: remove @console_locked

drivers/tty/sysrq.c | 2 +
include/linux/console.h | 19 +
include/linux/printk.h | 82 ++-
kernel/hung_task.c | 11 +-
kernel/panic.c | 4 +
kernel/printk/printk.c | 1197 +++++++++++++++++++++++++++++----------
kernel/rcu/tree_stall.h | 2 +
kernel/reboot.c | 14 +-
kernel/watchdog.c | 4 +
kernel/watchdog_hld.c | 4 +
lib/dump_stack.c | 4 +-
lib/nmi_backtrace.c | 4 +-
12 files changed, 1021 insertions(+), 326 deletions(-)

base-commit: 84d7df104dbab9c3dda8f2c5b46f9a6fc256fe02
--
2.30.2