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@xxxxxxxxxxxxx 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
