From: Thomas Gleixner Date: Tue, 20 Aug 2024 08:35:57 +0206 Subject: [PATCH 31/54] printk: nbcon: Implement emergency sections Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz In emergency situations (something has gone wrong but the system continues to operate), usually important information (such as a backtrace) is generated via printk(). This information should be pushed out to the consoles ASAP. Add per-CPU emergency nesting tracking because an emergency can arise while in an emergency situation. Add functions to mark the beginning and end of emergency sections where the urgent messages are generated. Perform direct console flushing at the emergency priority if the current CPU is in an emergency state and it is safe to do so. Note that the emergency state is not system-wide. While one CPU is in an emergency state, another CPU may attempt to print console messages at normal priority. Also note that printk() already attempts to flush consoles in the caller context for normal priority. However, follow-up changes will introduce printing kthreads, in which case the normal priority printk() calls will offload to the kthreads. Co-developed-by: John Ogness Signed-off-by: John Ogness Signed-off-by: Thomas Gleixner (Intel) Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/20240820063001.36405-32-john.ogness@linutronix.de Signed-off-by: Petr Mladek Signed-off-by: Sebastian Andrzej Siewior --- include/linux/console.h | 4 ++ kernel/printk/internal.h | 1 kernel/printk/nbcon.c | 75 ++++++++++++++++++++++++++++++++++++++++++++++- 3 files changed, 79 insertions(+), 1 deletion(-) --- a/include/linux/console.h +++ b/include/linux/console.h @@ -553,10 +553,14 @@ static inline bool console_is_registered hlist_for_each_entry(con, &console_list, node) #ifdef CONFIG_PRINTK +extern void nbcon_cpu_emergency_enter(void); +extern void nbcon_cpu_emergency_exit(void); extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt); extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt); extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt); #else +static inline void nbcon_cpu_emergency_enter(void) { } +static inline void nbcon_cpu_emergency_exit(void) { } static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return false; } static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; } static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; } --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -182,6 +182,7 @@ static inline void printk_get_console_fl switch (nbcon_get_default_prio()) { case NBCON_PRIO_NORMAL: + 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 @@ -972,6 +972,36 @@ static bool nbcon_emit_next_record(struc return nbcon_context_exit_unsafe(ctxt); } +/* 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; + +/** + * nbcon_get_cpu_emergency_nesting - Get the per CPU emergency nesting pointer + * + * Context: For reading, any context. For writing, any context which could + * not be migrated to another CPU. + * Return: Either a pointer to the per CPU emergency nesting counter of + * the current CPU or to the init data during early boot. + * + * The function is safe for reading per-CPU variables in any context because + * preemption is disabled if the current CPU is in the emergency state. See + * also nbcon_cpu_emergency_enter(). + */ +static __ref unsigned int *nbcon_get_cpu_emergency_nesting(void) +{ + /* + * The value of __printk_percpu_data_ready gets set in normal + * context and before SMP initialization. As a result it could + * never change while inside an nbcon emergency section. + */ + if (!printk_percpu_data_ready()) + return &early_nbcon_pcpu_emergency_nesting; + + /* Open code this_cpu_ptr() without checking migration. */ + return per_cpu_ptr(&nbcon_pcpu_emergency_nesting, raw_smp_processor_id()); +} + /** * nbcon_get_default_prio - The appropriate nbcon priority to use for nbcon * printing on the current CPU @@ -981,13 +1011,20 @@ static bool nbcon_emit_next_record(struc * context for printing. * * The function is safe for reading per-CPU data in any context because - * preemption is disabled if the current CPU is in the panic state. + * preemption is disabled if the current CPU is in the emergency or panic + * state. */ enum nbcon_prio nbcon_get_default_prio(void) { + unsigned int *cpu_emergency_nesting; + if (this_cpu_in_panic()) return NBCON_PRIO_PANIC; + cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting(); + if (*cpu_emergency_nesting) + return NBCON_PRIO_EMERGENCY; + return NBCON_PRIO_NORMAL; } @@ -1247,6 +1284,42 @@ void nbcon_atomic_flush_unsafe(void) } /** + * nbcon_cpu_emergency_enter - Enter an emergency section where printk() + * messages for that CPU are flushed directly + * + * Context: Any context. Disables preemption. + * + * When within an emergency section, printk() calls will attempt to flush any + * pending messages in the ringbuffer. + */ +void nbcon_cpu_emergency_enter(void) +{ + unsigned int *cpu_emergency_nesting; + + preempt_disable(); + + cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting(); + (*cpu_emergency_nesting)++; +} + +/** + * nbcon_cpu_emergency_exit - Exit an emergency section + * + * Context: Within an emergency section. Enables preemption. + */ +void nbcon_cpu_emergency_exit(void) +{ + unsigned int *cpu_emergency_nesting; + + cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting(); + + if (!WARN_ON_ONCE(*cpu_emergency_nesting == 0)) + (*cpu_emergency_nesting)--; + + preempt_enable(); +} + +/** * nbcon_alloc - Allocate and init the nbcon console specific data * @con: Console to initialize *