From 9de0e1ea58186532980a60aef430a8b385d572cb Mon Sep 17 00:00:00 2001 From: Thomas Gleixner <tglx@linutronix.de> Date: Fri, 22 Sep 2023 14:12:21 +0000 Subject: [PATCH 144/204] printk: nbcon: Introduce printing kthreads Provide the main implementation for running a printer kthread per nbcon console that is takeover/handover aware. The main print function nbcon_emit_next_record() will generate a warning if a task other than the dedicated printer thread tries to print using write_thread(). 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 | 8 ++ kernel/printk/internal.h | 25 ++++++ kernel/printk/nbcon.c | 188 ++++++++++++++++++++++++++++++++++++++++++++++- kernel/printk/printk.c | 31 +++++++ 4 files changed, 249 insertions(+), 3 deletions(-) Index: linux-6.6.58-rt45/include/linux/console.h =================================================================== @ linux-6.6.58-rt45/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-6.6.58-rt45/include/linux/console.h:300 @ struct nbcon_write_context { * @node: hlist node for the console list * * @write_atomic: Write callback for atomic context + * @write_thread: Write callback for non-atomic context * @driver_enter: Callback to begin synchronization with driver code * @driver_exit: Callback to finish synchronization with driver code * @nbcon_state: State for nbcon consoles * @nbcon_seq: Sequence number of the next record for nbcon to print * @pbufs: Pointer to nbcon private buffer * @locked_port: True, if the port lock is locked by nbcon + * @kthread: Printer kthread for this console + * @rcuwait: RCU-safe wait object for @kthread waking */ struct console { char name[16]; @ linux-6.6.58-rt45/include/linux/console.h:332 @ struct console { /* nbcon console specific members */ bool (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt); + bool (*write_thread)(struct console *con, + struct nbcon_write_context *wctxt); void (*driver_enter)(struct console *con, unsigned long *flags); void (*driver_exit)(struct console *con, unsigned long flags); atomic_t __private nbcon_state; atomic_long_t __private nbcon_seq; struct printk_buffers *pbufs; bool locked_port; + struct task_struct *kthread; + struct rcuwait rcuwait; }; #ifdef CONFIG_LOCKDEP Index: linux-6.6.58-rt45/kernel/printk/internal.h =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/internal.h +++ linux-6.6.58-rt45/kernel/printk/internal.h @ linux-6.6.58-rt45/include/linux/console.h:95 @ void nbcon_free(struct console *con); enum nbcon_prio nbcon_get_default_prio(void); void nbcon_atomic_flush_all(void); bool nbcon_atomic_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-6.6.58-rt45/include/linux/console.h:114 @ static inline bool console_is_usable(str if (flags & CON_NBCON) { if (!con->write_atomic) return false; + if (!con->write_thread || !con->kthread) + return false; } else { if (!con->write) return false; @ linux-6.6.58-rt45/include/linux/console.h:132 @ 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) { } #define printing_via_unlock (false) /* Index: linux-6.6.58-rt45/kernel/printk/nbcon.c =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/nbcon.c +++ linux-6.6.58-rt45/kernel/printk/nbcon.c @ linux-6.6.58-rt45/include/linux/console.h:8 @ #include <linux/kernel.h> #include <linux/console.h> #include <linux/delay.h> +#include <linux/kthread.h> #include <linux/slab.h> #include <linux/serial_core.h> +#include "printk_ringbuffer.h" #include "internal.h" /* * Printk console printing implementation for consoles which does not depend @ linux-6.6.58-rt45/include/linux/console.h:833 @ 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-6.6.58-rt45/include/linux/console.h:847 @ 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-6.6.58-rt45/include/linux/console.h:897 @ 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) { done = con->write_atomic(con, wctxt); + } else if (!use_atomic && + con->write_thread && + con->kthread) { + WARN_ON_ONCE(con->kthread != current); + done = con->write_thread(con, wctxt); + } + if (!done) { /* * The emit was aborted, probably due to a loss of ownership. @ linux-6.6.58-rt45/include/linux/console.h:943 @ 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 is_usable; + short flags; + int cookie; + + if (kthread_should_stop()) + return true; + + cookie = console_srcu_read_lock(); + flags = console_srcu_read_flags(con); + is_usable = console_is_usable(con, flags); + console_srcu_read_unlock(cookie); + + if (!is_usable) + return false; + + /* Bring the sequence in @ctxt up to date */ + ctxt->seq = nbcon_seq_read(con); + + return prb_read_valid(prb, ctxt->seq, NULL); +} + +/** + * 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->driver_enter(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->driver_exit(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-6.6.58-rt45/include/linux/console.h:1102 @ 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 false; nbcon_context_release(ctxt); @ linux-6.6.58-rt45/include/linux/console.h:1318 @ 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-6.6.58-rt45/include/linux/console.h:1420 @ 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, con->seq); nbcon_state_set(con, &state); } @ linux-6.6.58-rt45/include/linux/console.h:1433 @ 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-6.6.58-rt45/kernel/printk/printk.c =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/printk.c +++ linux-6.6.58-rt45/kernel/printk/printk.c @ linux-6.6.58-rt45/include/linux/console.h:2691 @ void suspend_console(void) void resume_console(void) { struct console *con; + short flags; + int cookie; if (!console_suspend_enabled) return; @ linux-6.6.58-rt45/include/linux/console.h:2709 @ 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-6.6.58-rt45/include/linux/console.h:3037 @ 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-6.6.58-rt45/include/linux/console.h:3339 @ 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);