Hi, This is v1 of a series to bring in a new threaded/atomic console infrastructure. The history, motivation, and various explanations and examples are available in the cover letter of tglx's RFC series [0]. From that series, patches 1-18 have been mainlined as of the 6.3 merge window. What remains, patches 19-29, is what this series represents. Since the RFC there has been significant changes involving bug-fixing, refactoring, renaming, and feature completion. Despite the many changes in the code, the concept and design has been preserved. The key points to the threaded/atomic (aka NOBKL) consoles are: - Each console has its own kthread and uses a new write_thread() console callback that is always called from sleepable context. The kthreads of different consoles do not contend with each other and do not use the global console lock. - Each console uses a new write_atomic() console callback that is able to write from any context (including NMI). The threaded/atomic infrastructure provides supporting functions to help atomic console drivers to synchronize with their own threaded and re-entrant atomic counterparts. - Until the console threads are available, atomic printing is performed. The threaded printing is able to take over shortly before SMP is brought online so machines with many CPUs should be able to boot at full speed without being held back by console printing. - When console threads are shutdown (on system reboot and shutdown), again the atomic consoles take over. This ensures the final message make it out to the consoles. - When panic, WARN, and NMI stall detection occurs, the atomic consoles temporarily take over printing until the related messages have been output. In this case the full set of related messages are stored into the printk ringbuffer before the atomic consoles begin flushing the ringbuffer to the consoles. - Atomic printing is split into 3 priorities. For example, upon shutdown (when kthreads are not available), the console output will be normal priority atomic printing. This could be interrupted by a WARN on another CPU (emergency priority). And that could be interrupted by a panic on yet another CPU (panic priority). And of course any atomic printing priority can interrupt the kthread printer. - The transition between kthread and any atomic printing or to any elevated priority atomic printing is synchronized using an atomic_t state variable per console. The state variable acts like a spinlock but with special properties that the spinning can timeout and even a hostile takeover based on the atomic priorities is possible. After outputting each byte, a console printing context checks the state variable to ensure it is still the owner of the console. If it is not (for example, in the case of a hostile takeover) it will immediately abort any continued use of the console and rely on the new owner to flush the messages. - Using the console state variable, console drivers can mark unsafe regions in their code where ownership transition is not possible. Combined with the timeout feature, a handover protocol, and the possibility for a hostile takeover, this allows drivers to make safe decisions about when and how console ownership is transferred to another context. It also allows the printk infrastructure to make safe decisions in panic situations, such as only outputting to atomic consoles where safe takeovers are possible. And only after handling all other panic responsibilities, attempting unsafe takeovers for the consoles that have not yet transferred ownership. - In order to support hostile takeovers (where a CPU with a higher priority context can steal ownership from another CPU) without CPUs clobbering each others buffers, each CPU has its own set of string print buffers. The existing legacy consoles continue to function unmodified as before and legacy consoles can work next to NOBKL consoles (i.e. a legacy virtual terminal graphics console and network console will work with a NOBKL uart8250 console). However, in order to have the full benefit/reliability of NOBKL consoles, a system should use _only_ NOBKL consoles. We believe that this series covers all printk features and usage to allow new threaded/atomic consoles to be able to replace the legacy consoles. However, this will be a gradual transition as individual console drivers are updated to support the NOBKL requirements. This series does not include any changes to console drivers to allow them to act as NOBKL consoles. That will be a follow-up series, once a finalized infrastructure is in place. However, I will reply to this message with an all-in-one uart8250 patch that fully implements NOBKL support. The patch will allow you to perform runtime tests with the NOBKL consoles on the uart8250. John Ogness [0] https://lore.kernel.org/lkml/20220910221947.171557773@xxxxxxxxxxxxx John Ogness (7): kdb: do not assume write() callback available printk: Add NMI check to down_trylock_console_sem() printk: Consolidate console deferred printing printk: Add per-console suspended state printk: nobkl: Stop threads on shutdown/reboot rcu: Add atomic write enforcement for rcu stalls printk: Perform atomic flush in console_flush_on_panic() Thomas Gleixner (11): printk: Add non-BKL console basic infrastructure printk: nobkl: Add acquire/release logic printk: nobkl: Add buffer management printk: nobkl: Add sequence handling printk: nobkl: Add print state functions printk: nobkl: Add emit function and callback functions for atomic printing printk: nobkl: Introduce printer threads printk: nobkl: Add printer thread wakeups printk: nobkl: Add write context storage for atomic writes printk: nobkl: Provide functions for atomic write enforcement kernel/panic: Add atomic write enforcement to warn/panic fs/proc/consoles.c | 1 + include/linux/console.h | 174 ++++ include/linux/printk.h | 9 + kernel/debug/kdb/kdb_io.c | 2 + kernel/panic.c | 17 + kernel/printk/Makefile | 2 +- kernel/printk/internal.h | 103 +- kernel/printk/printk.c | 307 ++++-- kernel/printk/printk_nobkl.c | 1820 ++++++++++++++++++++++++++++++++++ kernel/printk/printk_safe.c | 9 +- kernel/rcu/tree_stall.h | 6 + 11 files changed, 2362 insertions(+), 88 deletions(-) create mode 100644 kernel/printk/printk_nobkl.c base-commit: 10d639febe5629687dac17c4a7500a96537ce11a -- 2.30.2
