From: John Ogness Date: Wed, 4 Sep 2024 14:11:28 +0206 Subject: [PATCH 46/54] printk: nbcon: Rely on kthreads for normal operation Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz Once the kthread is running and available (i.e. @printk_kthreads_running is set), the kthread becomes responsible for flushing any pending messages which are added in NBCON_PRIO_NORMAL context. Namely the legacy console_flush_all() and device_release() no longer flush the console. And nbcon_atomic_flush_pending() used by nbcon_cpu_emergency_exit() no longer flushes messages added after the emergency messages. The console context is safe when used by the kthread only when one of the following conditions are true: 1. Other caller acquires the console context with NBCON_PRIO_NORMAL with preemption disabled. It will release the context before rescheduling. 2. Other caller acquires the console context with NBCON_PRIO_NORMAL under the device_lock. 3. The kthread is the only context which acquires the console with NBCON_PRIO_NORMAL. This is satisfied for all atomic printing call sites: nbcon_legacy_emit_next_record() (#1) nbcon_atomic_flush_pending_con() (#1) nbcon_device_release() (#2) It is even double guaranteed when @printk_kthreads_running is set because then _only_ the kthread will print for NBCON_PRIO_NORMAL. (#3) Signed-off-by: John Ogness Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/20240904120536.115780-10-john.ogness@linutronix.de Signed-off-by: Petr Mladek Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/internal.h | 26 +++++++++++++++++++++++++ kernel/printk/nbcon.c | 17 ++++++++++------ kernel/printk/printk.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++- 3 files changed, 84 insertions(+), 7 deletions(-) --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -113,6 +113,13 @@ static inline bool console_is_usable(str /* The write_atomic() callback is optional. */ if (use_atomic && !con->write_atomic) return false; + + /* + * For the !use_atomic case, @printk_kthreads_running is not + * checked because the write_thread() callback is also used + * via the legacy loop when the printer threads are not + * available. + */ } else { if (!con->write) return false; @@ -176,6 +183,7 @@ static inline void nbcon_atomic_flush_pe static inline bool nbcon_legacy_emit_next_record(struct console *con, bool *handover, int cookie, bool use_atomic) { return false; } static inline void nbcon_kthread_wake(struct console *con) { } +static inline void nbcon_kthreads_wake(void) { } static inline bool console_is_usable(struct console *con, short flags, bool use_atomic) { return false; } @@ -190,6 +198,7 @@ extern bool legacy_allow_panic_sync; /** * struct console_flush_type - Define available console flush methods * @nbcon_atomic: Flush directly using nbcon_atomic() callback + * @nbcon_offload: Offload flush to printer thread * @legacy_direct: Call the legacy loop in this context * @legacy_offload: Offload the legacy loop into IRQ * @@ -197,6 +206,7 @@ extern bool legacy_allow_panic_sync; */ struct console_flush_type { bool nbcon_atomic; + bool nbcon_offload; bool legacy_direct; bool legacy_offload; }; @@ -211,6 +221,22 @@ static inline void printk_get_console_fl switch (nbcon_get_default_prio()) { case NBCON_PRIO_NORMAL: + if (have_nbcon_console && !have_boot_console) { + if (printk_kthreads_running) + ft->nbcon_offload = true; + else + ft->nbcon_atomic = true; + } + + /* Legacy consoles are flushed directly when possible. */ + if (have_legacy_console || have_boot_console) { + if (!is_printk_legacy_deferred()) + ft->legacy_direct = true; + else + ft->legacy_offload = true; + } + break; + case NBCON_PRIO_EMERGENCY: if (have_nbcon_console && !have_boot_console) ft->nbcon_atomic = true; --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -1494,6 +1494,7 @@ static int __nbcon_atomic_flush_pending_ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq, bool allow_unsafe_takeover) { + struct console_flush_type ft; unsigned long flags; int err; @@ -1523,10 +1524,12 @@ static void nbcon_atomic_flush_pending_c /* * If flushing was successful but more records are available, this - * context must flush those remaining records because there is no - * other context that will do it. + * context must flush those remaining records if the printer thread + * is not available do it. */ - if (prb_read_valid(prb, nbcon_seq_read(con), NULL)) { + printk_get_console_flush_type(&ft); + if (!ft.nbcon_offload && + prb_read_valid(prb, nbcon_seq_read(con), NULL)) { stop_seq = prb_next_reserve_seq(prb); goto again; } @@ -1754,17 +1757,19 @@ void nbcon_device_release(struct console /* * This context must flush any new records added while the console - * was locked. The console_srcu_read_lock must be taken to ensure - * the console is usable throughout flushing. + * was locked if the printer thread is not available to do it. The + * console_srcu_read_lock must be taken to ensure the console is + * usable throughout flushing. */ cookie = console_srcu_read_lock(); + printk_get_console_flush_type(&ft); if (console_is_usable(con, console_srcu_read_flags(con), true) && + !ft.nbcon_offload && prb_read_valid(prb, nbcon_seq_read(con), NULL)) { /* * If nbcon_atomic flushing is not available, fallback to * using the legacy loop. */ - printk_get_console_flush_type(&ft); if (ft.nbcon_atomic) { __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false); } else if (ft.legacy_direct) { --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2384,6 +2384,9 @@ asmlinkage int vprintk_emit(int facility if (ft.nbcon_atomic) nbcon_atomic_flush_pending(); + if (ft.nbcon_offload) + nbcon_kthreads_wake(); + if (ft.legacy_direct) { /* * The caller may be holding system-critical or @@ -2733,6 +2736,7 @@ void suspend_console(void) void resume_console(void) { + struct console_flush_type ft; struct console *con; if (!console_suspend_enabled) @@ -2750,6 +2754,10 @@ void resume_console(void) */ synchronize_srcu(&console_srcu); + printk_get_console_flush_type(&ft); + if (ft.nbcon_offload) + nbcon_kthreads_wake(); + pr_flush(1000, true); } @@ -3061,6 +3069,7 @@ static inline void printk_kthreads_check */ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover) { + struct console_flush_type ft; bool any_usable = false; struct console *con; bool any_progress; @@ -3072,12 +3081,22 @@ static bool console_flush_all(bool do_co do { any_progress = false; + printk_get_console_flush_type(&ft); + cookie = console_srcu_read_lock(); for_each_console_srcu(con) { short flags = console_srcu_read_flags(con); u64 printk_seq; bool progress; + /* + * console_flush_all() is only responsible for nbcon + * consoles when the nbcon consoles cannot print via + * their atomic or threaded flushing. + */ + if ((flags & CON_NBCON) && (ft.nbcon_atomic || ft.nbcon_offload)) + continue; + if (!console_is_usable(con, flags, !do_cond_resched)) continue; any_usable = true; @@ -3388,9 +3407,25 @@ EXPORT_SYMBOL(console_stop); void console_start(struct console *console) { + struct console_flush_type ft; + bool is_nbcon; + console_list_lock(); console_srcu_write_flags(console, console->flags | CON_ENABLED); + is_nbcon = console->flags & CON_NBCON; 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); + + printk_get_console_flush_type(&ft); + if (is_nbcon && ft.nbcon_offload) + nbcon_kthread_wake(console); + __pr_flush(console, 1000, true); } EXPORT_SYMBOL(console_start); @@ -4116,6 +4151,8 @@ static bool __pr_flush(struct console *c /* Flush the consoles so that records up to @seq are printed. */ printk_get_console_flush_type(&ft); + if (ft.nbcon_atomic) + nbcon_atomic_flush_pending(); if (ft.legacy_direct) { console_lock(); console_unlock(); @@ -4153,8 +4190,10 @@ static bool __pr_flush(struct console *c * that they make forward progress, so only increment * @diff for usable consoles. */ - if (!console_is_usable(c, flags, true)) + if (!console_is_usable(c, flags, true) && + !console_is_usable(c, flags, false)) { continue; + } if (flags & CON_NBCON) { printk_seq = nbcon_seq_read(c); @@ -4630,8 +4669,15 @@ EXPORT_SYMBOL_GPL(kmsg_dump_rewind); */ void console_try_replay_all(void) { + struct console_flush_type ft; + + printk_get_console_flush_type(&ft); if (console_trylock()) { __console_rewind_all(); + if (ft.nbcon_atomic) + nbcon_atomic_flush_pending(); + if (ft.nbcon_offload) + nbcon_kthreads_wake(); /* Consoles are flushed as part of console_unlock(). */ console_unlock(); }