From: Jan Kara <j...@suse.cz> Necessity for offloading of printing was observed only for large systems. So add a config option (disabled by default) which removes most of the overhead added by this functionality.
Signed-off-by: Jan Kara <j...@suse.cz> --- Documentation/kernel-parameters.txt | 13 +++++++------ init/Kconfig | 14 ++++++++++++++ kernel/printk/printk.c | 21 +++++++++++++++++++++ 3 files changed, 42 insertions(+), 6 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 4e0d2d8c0f97..0ed7f0f1d87a 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2943,18 +2943,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted. Format: <bool> (1/Y/y=enable, 0/N/n=disable) default: disabled - printk.offload_chars= + printk.offload_chars= [KNL] Printing to console can be relatively slow especially in case of serial console. When there is intensive printing happening from several cpus (as is the case during boot), a cpu can be spending significant time (seconds or more) doing printing. To avoid softlockups, lost interrupts, and similar problems other cpus - will take over printing after the currently printing - cpu has printed 'printk.offload_chars' characters. - Higher value means possibly longer interrupt and other - latencies but lower overhead of printing due to handing - over of printing. + will take over printing (if CONFIG_PRINTK_OFFLOAD=y) + after the currently printing cpu has printed + 'printk.offload_chars' characters. Higher value means + possibly longer interrupt and other latencies but + lower overhead of printing due to handing over of + printing. Format: <number> (0 = disabled) default: 1000 diff --git a/init/Kconfig b/init/Kconfig index af09b4fb43d2..71b296483d0d 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1439,6 +1439,20 @@ config PRINTK very difficult to diagnose system problems, saying N here is strongly discouraged. +config PRINTK_OFFLOAD + default y + bool "Enable support for offloading printing to different CPU" + depends on PRINTK + help + Printing to console can be relatively slow especially in case of + serial console. On large machines when there is intensive printing + happening from several cpus (as is the case during boot), a cpu can + be spending significant time (seconds or more) doing printing. To + avoid softlockups, lost interrupts, and similar problems other cpus + will take over printing after the currently printing cpu has printed + certain number of characters (tunable via 'printk.offload_chars' + kernel parameter). + config BUG bool "BUG() support" if EXPERT default y diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index c443f1aa7e84..63d78ab19278 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -79,6 +79,7 @@ static DEFINE_SEMAPHORE(console_sem); struct console *console_drivers; EXPORT_SYMBOL_GPL(console_drivers); +#ifdef CONFIG_PRINTK_OFFLOAD /* * This spinlock is taken when printing to console. It is used only so that * we can spin on it when some other thread wants to take over printing to @@ -105,6 +106,7 @@ static DEFINE_MUTEX(printk_kthread_mutex); /* Wait queue printing kthreads sleep on when idle */ static DECLARE_WAIT_QUEUE_HEAD(print_queue); +#endif /* CONFIG_PRINTK_OFFLOAD */ #ifdef CONFIG_LOCKDEP static struct lockdep_map console_lock_dep_map = { @@ -308,6 +310,7 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); static char *log_buf = __log_buf; static u32 log_buf_len = __LOG_BUF_LEN; +#ifdef CONFIG_PRINTK_OFFLOAD static int offload_chars_set(const char *val, const struct kernel_param *kp); static struct kernel_param_ops offload_chars_ops = { .set = offload_chars_set, @@ -326,6 +329,7 @@ module_param_cb(offload_chars, &offload_chars_ops, &printk_offload_chars, S_IRUGO | S_IWUSR); MODULE_PARM_DESC(offload_chars, "offload printing to console to a different" " cpu after this number of characters"); +#endif /* Return log buffer address */ char *log_buf_addr_get(void) @@ -2255,6 +2259,7 @@ out: raw_spin_unlock_irqrestore(&logbuf_lock, flags); } +#ifdef CONFIG_PRINTK_OFFLOAD /* * Returns true iff there is other cpu waiting to take over printing. This * function also takes are of setting PRINTK_HANDOVER_B if we want to hand over @@ -2278,6 +2283,14 @@ static bool cpu_stop_printing(int printed_chars) return false; } +#else + +static bool cpu_stop_printing(int printed_chars, bool *woken) +{ + return false; +} + +#endif /** * console_unlock - unlock the console system @@ -2316,7 +2329,9 @@ void console_unlock(void) /* flush buffered message fragment immediately to console */ console_cont_flush(text, sizeof(text)); again: +#ifdef CONFIG_PRINTK_OFFLOAD spin_lock(&print_lock); +#endif for (;;) { struct printk_log *msg; size_t ext_len = 0; @@ -2399,12 +2414,14 @@ skip: console_locked = 0; up_console_sem(); +#ifdef CONFIG_PRINTK_OFFLOAD /* * Release print_lock after console_sem so that printing_task() * succeeds in getting console_sem (unless someone else takes it and * then he'll be responsible for printing). */ spin_unlock(&print_lock); +#endif /* * Subtlety: We have interrupts disabled iff hand_over == false (to @@ -2770,6 +2787,7 @@ int unregister_console(struct console *console) } EXPORT_SYMBOL(unregister_console); +#ifdef CONFIG_PRINTK_OFFLOAD /* Kthread which takes over printing from a CPU which asks for help */ static int printing_task(void *arg) { @@ -2838,6 +2856,7 @@ static int offload_chars_set(const char *val, const struct kernel_param *kp) mutex_unlock(&printk_kthread_mutex); return 0; } +#endif /* CONFIG_PRINTK_OFFLOAD */ static int __init printk_late_init(void) { @@ -2850,9 +2869,11 @@ static int __init printk_late_init(void) } hotcpu_notifier(console_cpu_notify, 0); +#ifdef CONFIG_PRINTK_OFFLOAD mutex_lock(&printk_kthread_mutex); printk_start_offload_kthreads(); mutex_unlock(&printk_kthread_mutex); +#endif return 0; } -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/