This is v4 of a series to implement a kthread for each registered console. v3 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/20220419234637.357112-1-john.ogness@xxxxxxxxxxxxx Changes since v3: - For defer_console_output(), call allow_direct_printing() instead of only checking @printk_prefer_direct. - Remove console_lock_single_hold() and console_unlock_single_release() functions. Use the console_lock for console_stop(), console_start(), unregister_console(), printk_kthread_func(). - Introduce macros console_flags_set() and console_flags_clear() to adjust con->flags using READ_ONCE()/WRITE_ONCE() in order to guarantee consistent values for the variable. (This does not make the RMW operations atomic, but the console_lock and con->lock are still used to synchronize between tasks that modify con->flags.) - Add and/or expand comments for allow_direct_printing(), console_cpu_notify(), __console_unlock(), console_stop(), console_start(), unregister_console(), printk_kthread_func(), defer_console_output(). 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 | 1234 +++++++++++++++++++++++++++++---------- 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, 1059 insertions(+), 325 deletions(-) base-commit: 84d7df104dbab9c3dda8f2c5b46f9a6fc256fe02 -- 2.30.2
