This is v2 of a series to implement a kthread for each registered console. v1 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, it completely decouples printk() callers 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. Changes since v1: - Rename nearly all new variables, functions, macros, and flags: variables: @console_lock_count --> @console_kthreads_active @consoles_paused --> @console_kthreads_blocked @kthreads_started --> @printk_kthreads_available @printk_direct --> @printk_prefer_direct functions/macros: console_excl_trylock() --> console_kthreads_atomic_tryblock() console_excl_unlock() --> console_kthreads_atomic_unblock() console_printer_tryenter() --> console_kthread_printing_tryenter() console_printer_exit() --> console_kthread_printing_exit() pause_all_consoles() --> console_kthreads_block() unpause_all_consoles() --> console_kthreads_unblock() printk_direct_enter() --> printk_prefer_direct_enter() printk_direct_exit() --> printk_prefer_direct_exit() start_printk_kthread() --> printk_start_kthread() console flags: CON_PAUSED --> CON_THD_BLOCKED - Drop the patch to avoid VT printing if in an atomic context. (This can only be done after the Linux BSoD feature is implemented.) - Keep printk_delay() in the printk() context. (We may want to move this into printer contexts later.) - Keep console_trylock() usage to reacquire the console lock in console_unlock(). - Keep cpu_online(raw_smp_processor_id()) in the check if a console is usable and remove the printk_percpu_data_ready() check (percpu data is always ready for consoles). - Start console printer threads as an early_initcall. This means threads are active before multiple CPUs come online. - Remove "system_state < SYSTEM_RUNNING" as a reason to allow direct printing. @kthreads_started is used for the early boot decision. - Add console_kthreads_atomically_blocked() macro. - Add printk_console_msg() macro to printk messages with console details. The macro allows print indexing. - Add __pr_flush() to allow waiting for only a single console. - Add a full memory barrier in wake_up_klogd() to ensure new records are visible before checking if the waitqueue is empty. - Wake kthreads when exiting preferred direct mode. - Wake kthreads when a CPU comes online. - In unregister_console(), stop the printer thread before acquiring the console lock. - Print "thread started" message after all allocations successful. - Permanently enable preferred direct printing if there are any errors setting up any of the threaded console printers. - Keep threaded printers asleep if there is no con->write() callback. - Keep threaded printers asleep if preferred direct printing is active. - Always allow handovers if the console is locked. - Only stop irqsoff tracing if handovers are allowed. - console_emit_next_record() is for when the console is not locked, console_emit_next_record_transferable() is for when the console is locked. - Wait for console printers to catch up after unblanking the screen. - In console_stop(), only wait for the stopping console. - In console_start(), only wait for the starting console. - Print only 1 record per wait cycle to increase chances of direct printing taking over. - Repurpose the irqwork flag PRINTK_PENDING_OUTPUT for direct printing. - Add caution notice and tips to description of printk_cpu_sync_get_irqsave() on how to use it correctly. - Improve the comments explaining why preemption is disabled during direct printing. - Improve commit message to explain the motivation for expanding the console lock and how it works. John Ogness [0] https://lore.kernel.org/all/20220207194323.273637-1-john.ogness@xxxxxxxxxxxxx John Ogness (12): printk: rename cpulock functions printk: cpu sync always disable interrupts printk: get caller_id/timestamp after migration disable printk: call boot_delay_msec() in printk_delay() printk: add macro for console detail messages 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 | 1070 +++++++++++++++++++++++++++++---------- 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, 908 insertions(+), 312 deletions(-) base-commit: 3ef4ea3d84ca568dcd57816b9521e82e3bd94f08 -- 2.30.2
