From: Thomas Gleixner <tglx@linutronix.de> Date: Fri, 22 Sep 2023 14:12:21 +0000 Subject: [PATCH 33/48] printk: nbcon: Introduce printing kthreads Provide the main implementation for running a printer kthread per nbcon console that is takeover/handover aware. Co-developed-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- include/linux/console.h | 26 ++++++ kernel/printk/internal.h | 28 +++++++ kernel/printk/nbcon.c | 187 +++++++++++++++++++++++++++++++++++++++++++++-- kernel/printk/printk.c | 33 ++++++++ 4 files changed, 269 insertions(+), 5 deletions(-) Index: linux-6.8.2-rt10/include/linux/console.h =================================================================== @ linux-6.8.2-rt10/include/linux/console.h:20 @ #include <linux/atomic.h> #include <linux/bits.h> #include <linux/rculist.h> +#include <linux/rcuwait.h> #include <linux/types.h> struct vc_data; @ linux-6.8.2-rt10/include/linux/console.h:328 @ struct nbcon_drvdata { * @nbcon_state: State for nbcon consoles * @nbcon_seq: Sequence number of the next record for nbcon to print * @pbufs: Pointer to nbcon private buffer + * @kthread: Printer kthread for this console + * @rcuwait: RCU-safe wait object for @kthread waking */ struct console { char name[16]; @ linux-6.8.2-rt10/include/linux/console.h:378 @ struct console { void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt); /** + * @write_thread: + * + * NBCON callback to write out text in task context. (Optional) + * + * This callback is called with the console already acquired. Any + * additional driver synchronization should have been performed by + * device_lock(). + * + * This callback is always called from task context but with migration + * disabled. + * + * The same criteria for console ownership verification and unsafe + * sections applies as with write_atomic(). The difference between + * this callback and write_atomic() is that this callback is used + * during normal operation and is always called from task context. + * This provides drivers with a relatively relaxed locking context + * for synchronizing output to the hardware. + */ + void (*write_thread)(struct console *con, struct nbcon_write_context *wctxt); + + /** * @device_lock: * * NBCON callback to begin synchronization with driver code. @ linux-6.8.2-rt10/include/linux/console.h:458 @ struct console { struct nbcon_drvdata *nbcon_drvdata; struct printk_buffers *pbufs; + struct task_struct *kthread; + struct rcuwait rcuwait; }; #ifdef CONFIG_LOCKDEP Index: linux-6.8.2-rt10/kernel/printk/internal.h =================================================================== --- linux-6.8.2-rt10.orig/kernel/printk/internal.h +++ linux-6.8.2-rt10/kernel/printk/internal.h @ linux-6.8.2-rt10/include/linux/console.h:93 @ enum nbcon_prio nbcon_get_default_prio(v void nbcon_atomic_flush_pending(void); bool nbcon_legacy_emit_next_record(struct console *con, bool *handover, int cookie); +void nbcon_kthread_create(struct console *con); /* * Check if the given console is currently capable and allowed to print @ linux-6.8.2-rt10/include/linux/console.h:112 @ static inline bool console_is_usable(str if (flags & CON_NBCON) { if (!con->write_atomic) return false; + if (!con->write_thread) + return false; } else { if (!con->write) return false; @ linux-6.8.2-rt10/include/linux/console.h:130 @ static inline bool console_is_usable(str return true; } +/** + * nbcon_kthread_wake - Wake up a printk thread + * @con: Console to operate on + */ +static inline void nbcon_kthread_wake(struct console *con) +{ + /* + * Guarantee any new records can be seen by tasks preparing to wait + * before this context checks if the rcuwait is empty. + * + * The full memory barrier in rcuwait_wake_up() pairs with the full + * memory barrier within set_current_state() of + * ___rcuwait_wait_event(), which is called after prepare_to_rcuwait() + * adds the waiter but before it has checked the wait condition. + * + * This pairs with nbcon_kthread_func:A. + */ + rcuwait_wake_up(&con->rcuwait); /* LMM(nbcon_kthread_wake:A) */ +} + #else #define PRINTK_PREFIX_MAX 0 #define PRINTK_MESSAGE_MAX 0 #define PRINTKRB_RECORD_MAX 0 +static inline void nbcon_kthread_wake(struct console *con) { } +static inline void nbcon_kthread_create(struct console *con) { } + /* * In !PRINTK builds we still export console_sem * semaphore and some of console functions (console_unlock()/etc.), so @ linux-6.8.2-rt10/include/linux/console.h:182 @ static inline bool console_is_usable(str #endif /* CONFIG_PRINTK */ +extern bool have_boot_console; + extern struct printk_buffers printk_shared_pbufs; /** Index: linux-6.8.2-rt10/kernel/printk/nbcon.c =================================================================== --- linux-6.8.2-rt10.orig/kernel/printk/nbcon.c +++ linux-6.8.2-rt10/kernel/printk/nbcon.c @ linux-6.8.2-rt10/include/linux/console.h:13 @ #include <linux/export.h> #include <linux/init.h> #include <linux/irqflags.h> +#include <linux/kthread.h> #include <linux/minmax.h> #include <linux/percpu.h> #include <linux/preempt.h> @ linux-6.8.2-rt10/include/linux/console.h:841 @ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe); /** * nbcon_emit_next_record - Emit a record in the acquired context * @wctxt: The write context that will be handed to the write function + * @use_atomic: True if the write_atomic callback is to be used * * Return: True if this context still owns the console. False if * ownership was handed over or taken. @ linux-6.8.2-rt10/include/linux/console.h:855 @ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe); * When true is returned, @wctxt->ctxt.backlog indicates whether there are * still records pending in the ringbuffer, */ -static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt) +static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_atomic) { struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt); struct console *con = ctxt->console; @ linux-6.8.2-rt10/include/linux/console.h:904 @ static bool nbcon_emit_next_record(struc nbcon_state_read(con, &cur); wctxt->unsafe_takeover = cur.unsafe_takeover; - if (con->write_atomic) { + if (use_atomic && + con->write_atomic) { con->write_atomic(con, wctxt); + + } else if (!use_atomic && + con->write_thread) { + con->write_thread(con, wctxt); + } else { /* * This function should never be called for legacy consoles. @ linux-6.8.2-rt10/include/linux/console.h:947 @ update_con: return nbcon_context_exit_unsafe(ctxt); } +/** + * nbcon_kthread_should_wakeup - Check whether a printer thread should wakeup + * @con: Console to operate on + * @ctxt: The acquire context that contains the state + * at console_acquire() + * + * Return: True if the thread should shutdown or if the console is + * allowed to print and a record is available. False otherwise. + * + * After the thread wakes up, it must first check if it should shutdown before + * attempting any printing. + */ +static bool nbcon_kthread_should_wakeup(struct console *con, struct nbcon_context *ctxt) +{ + bool ret = false; + short flags; + int cookie; + + if (kthread_should_stop()) + return true; + + cookie = console_srcu_read_lock(); + + flags = console_srcu_read_flags(con); + if (console_is_usable(con, flags)) { + /* Bring the sequence in @ctxt up to date */ + ctxt->seq = nbcon_seq_read(con); + + ret = prb_read_valid(prb, ctxt->seq, NULL); + } + + console_srcu_read_unlock(cookie); + return ret; +} + +/** + * nbcon_kthread_func - The printer thread function + * @__console: Console to operate on + */ +static int nbcon_kthread_func(void *__console) +{ + struct console *con = __console; + struct nbcon_write_context wctxt = { + .ctxt.console = con, + .ctxt.prio = NBCON_PRIO_NORMAL, + }; + struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt); + unsigned long flags; + short con_flags; + bool backlog; + int cookie; + int ret; + +wait_for_event: + /* + * Guarantee this task is visible on the rcuwait before + * checking the wake condition. + * + * The full memory barrier within set_current_state() of + * ___rcuwait_wait_event() pairs with the full memory + * barrier within rcuwait_has_sleeper(). + * + * This pairs with rcuwait_has_sleeper:A and nbcon_kthread_wake:A. + */ + ret = rcuwait_wait_event(&con->rcuwait, + nbcon_kthread_should_wakeup(con, ctxt), + TASK_INTERRUPTIBLE); /* LMM(nbcon_kthread_func:A) */ + + if (kthread_should_stop()) + return 0; + + /* Wait was interrupted by a spurious signal, go back to sleep. */ + if (ret) + goto wait_for_event; + + do { + backlog = false; + + cookie = console_srcu_read_lock(); + + con_flags = console_srcu_read_flags(con); + + if (console_is_usable(con, con_flags)) { + con->device_lock(con, &flags); + + /* + * Ensure this stays on the CPU to make handover and + * takeover possible. + */ + cant_migrate(); + + if (nbcon_context_try_acquire(ctxt)) { + /* + * If the emit fails, this context is no + * longer the owner. + */ + if (nbcon_emit_next_record(&wctxt, false)) { + nbcon_context_release(ctxt); + backlog = ctxt->backlog; + } + } + + con->device_unlock(con, flags); + } + + console_srcu_read_unlock(cookie); + + } while (backlog); + + goto wait_for_event; +} + /* Track the nbcon emergency nesting per CPU. */ static DEFINE_PER_CPU(unsigned int, nbcon_pcpu_emergency_nesting); static unsigned int early_nbcon_pcpu_emergency_nesting __initdata; @ linux-6.8.2-rt10/include/linux/console.h:1105 @ static bool nbcon_atomic_emit_one(struct * handed over or taken over. In both cases the context is no * longer valid. */ - if (!nbcon_emit_next_record(wctxt)) + if (!nbcon_emit_next_record(wctxt, true)) return true; nbcon_context_release(ctxt); @ linux-6.8.2-rt10/include/linux/console.h:1214 @ static bool __nbcon_atomic_flush_pending * handed over or taken over. In both cases the context is no * longer valid. */ - if (!nbcon_emit_next_record(&wctxt)) + if (!nbcon_emit_next_record(&wctxt, true)) return true; if (!ctxt->backlog) @ linux-6.8.2-rt10/include/linux/console.h:1353 @ void nbcon_cpu_emergency_exit(void) } /** + * nbcon_kthread_stop - Stop a printer thread + * @con: Console to operate on + */ +static void nbcon_kthread_stop(struct console *con) +{ + lockdep_assert_console_list_lock_held(); + + if (!con->kthread) + return; + + kthread_stop(con->kthread); + con->kthread = NULL; +} + +/** + * nbcon_kthread_create - Create a printer thread + * @con: Console to operate on + * + * If it fails, let the console proceed. The atomic part might + * be usable and useful. + */ +void nbcon_kthread_create(struct console *con) +{ + struct task_struct *kt; + + lockdep_assert_console_list_lock_held(); + + if (!(con->flags & CON_NBCON) || !con->write_thread) + return; + + if (con->kthread) + return; + + /* + * Printer threads cannot be started as long as any boot console is + * registered because there is no way to synchronize the hardware + * registers between boot console code and regular console code. + */ + if (have_boot_console) + return; + + kt = kthread_run(nbcon_kthread_func, con, "pr/%s%d", con->name, con->index); + if (IS_ERR(kt)) { + con_printk(KERN_ERR, con, "failed to start printing thread\n"); + return; + } + + con->kthread = kt; + + /* + * It is important that console printing threads are scheduled + * shortly after a printk call and with generous runtime budgets. + */ + sched_set_normal(con->kthread, -20); +} + +/** * nbcon_alloc - Allocate buffers needed by the nbcon console * @con: Console to allocate buffers for * @ linux-6.8.2-rt10/include/linux/console.h:1453 @ void nbcon_init(struct console *con) /* nbcon_alloc() must have been called and successful! */ BUG_ON(!con->pbufs); + rcuwait_init(&con->rcuwait); nbcon_seq_force(con, 0); nbcon_state_set(con, &state); } @ linux-6.8.2-rt10/include/linux/console.h:1466 @ void nbcon_free(struct console *con) { struct nbcon_state state = { }; + nbcon_kthread_stop(con); nbcon_state_set(con, &state); /* Boot consoles share global printk buffers. */ Index: linux-6.8.2-rt10/kernel/printk/printk.c =================================================================== --- linux-6.8.2-rt10.orig/kernel/printk/printk.c +++ linux-6.8.2-rt10/kernel/printk/printk.c @ linux-6.8.2-rt10/include/linux/console.h:486 @ static bool have_nbcon_console; * the console lock. This is because boot consoles and nbcon consoles may * have mapped the same hardware. */ -static bool have_boot_console; +bool have_boot_console; /* * Specifies if the console lock/unlock dance is needed for console @ linux-6.8.2-rt10/include/linux/console.h:2701 @ void suspend_console(void) void resume_console(void) { struct console *con; + short flags; + int cookie; if (!console_suspend_enabled) return; @ linux-6.8.2-rt10/include/linux/console.h:2719 @ void resume_console(void) */ synchronize_srcu(&console_srcu); + cookie = console_srcu_read_lock(); + for_each_console_srcu(con) { + flags = console_srcu_read_flags(con); + if (flags & CON_NBCON) + nbcon_kthread_wake(con); + } + console_srcu_read_unlock(cookie); + pr_flush(1000, true); } @ linux-6.8.2-rt10/include/linux/console.h:3047 @ static bool console_flush_all(bool do_co u64 printk_seq; bool progress; + /* + * console_flush_all() is only for legacy consoles, + * unless the nbcon console has no kthread printer. + */ + if ((flags & CON_NBCON) && con->kthread) + continue; + if (!console_is_usable(con, flags)) continue; any_usable = true; @ linux-6.8.2-rt10/include/linux/console.h:3347 @ EXPORT_SYMBOL(console_stop); void console_start(struct console *console) { + short flags; + console_list_lock(); console_srcu_write_flags(console, console->flags | CON_ENABLED); + flags = console->flags; console_list_unlock(); + + /* + * Ensure that all SRCU list walks have completed. The related + * printing context must be able to see it is enabled so that + * it is guaranteed to wake up and resume printing. + */ + synchronize_srcu(&console_srcu); + + if (flags & CON_NBCON) + nbcon_kthread_wake(console); + __pr_flush(console, 1000, true); } EXPORT_SYMBOL(console_start);