On (03/07/16 13:16), Jan Kara wrote: [..] > > So for UP systems, we should by default disable async printing anyway I > > suppose. It is just a pointless overhead. So please just make printk_sync > > default to true if !CONFIG_SMP. > > > > When IRQs are disabled, you're right we will have a change in behavior. I > > don't see an easy way of avoiding delaying of printk until IRQs get > > enabled. I don't want to queue work directly because that creates > > possibility for lock recursion in queue_work(). And playing some tricks > > with irq_works isn't easy either - you cannot actually rely on any other > > CPU doing anything (even a timer tick) because of NOHZ. > > > > So if this will be a problem in practice, using a kthread will probably be > > the easiest solution. > > Hum, and thinking more about it: Considering that WQ_MEM_RECLAIM workqueues > create kthread anyway as a rescuer thread, it may be the simplest to just > go back to using a single kthread for printing. What do you think?
A new version. Switched to [printk] kthread. Minimally tested only. ===8<===8<=== >From 6f697488b20b356e4103ddcc4f8b0ec1fa6a9531 Mon Sep 17 00:00:00 2001 From: Jan Kara <j...@suse.cz> Date: Mon, 7 Mar 2016 23:53:46 +0900 Subject: [PATCH 1/2] printk: Make printk() completely async Currently, printk() sometimes waits for message to be printed to console and sometimes it does not (when console_sem is held by some other process). In case printk() grabs console_sem and starts printing to console, it prints messages from kernel printk buffer until the buffer is empty. When serial console is attached, printing is slow and thus other CPUs in the system have plenty of time to append new messages to the buffer while one CPU is printing. Thus the CPU can spend unbounded amount of time doing printing in console_unlock(). This is especially serious problem if the printk() calling console_unlock() was called with interrupts disabled. In practice users have observed a CPU can spend tens of seconds printing in console_unlock() (usually during boot when hundreds of SCSI devices are discovered) resulting in RCU stalls (CPU doing printing doesn't reach quiescent state for a long time), softlockup reports (IPIs for the printing CPU don't get served and thus other CPUs are spinning waiting for the printing CPU to process IPIs), and eventually a machine death (as messages from stalls and lockups append to printk buffer faster than we are able to print). So these machines are unable to boot with serial console attached. Another observed issue is that due to slow printk, hardware discovery is slow and udev times out before kernel manages to discover all the attached HW. Also during artificial stress testing SATA disk disappears from the system because its interrupts aren't served for too long. This patch makes printk() completely asynchronous (similar to what printk_deferred() did until now). It appends message to the kernel printk buffer and queues work to do the printing to console. This has the advantage that printing always happens from a schedulable contex and thus we don't lockup any particular CPU or even interrupts. Also it has the advantage that printk() is fast and thus kernel booting is not slowed down by slow serial console. Disadvantage of this method is that in case of crash there is higher chance that important messages won't appear in console output (we may need working scheduling to print message to console). We somewhat mitigate this risk by switching printk to the original method of immediate printing to console if oops is in progress. Also for debugging purposes we provide printk.synchronous kernel parameter which resorts to the original printk behavior. printk() is expected to work under different conditions and in different scenarios, including corner cases of OOM when all of the workers are busy (e.g. allocating memory), thus printk() uses its own dedicated printing kthread, rather than relying on workqueue (even with WQ_MEM_RECLAIM bit set we potentially can receive delays in printing until workqueue declares a ->mayday, as noted by Tetsuo Handa). Another thing to mention, is that deferred printk() messages may appear before @printk_thread is allocated, so in the very beginning we have to print deferred messages the old way -- via IRQs. Signed-off-by: Jan Kara <j...@suse.cz> Signed-off-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com> --- Documentation/kernel-parameters.txt | 10 ++ kernel/printk/printk.c | 193 +++++++++++++++++++++++++----------- 2 files changed, 145 insertions(+), 58 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index e0a21e4..5b01941 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -3108,6 +3108,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted. printk.time= Show timing data prefixed to each printk message line Format: <bool> (1/Y/y=enable, 0/N/n=disable) + printk.synchronous= + By default kernel messages are printed to console + asynchronously (except during early boot or when oops + is happening). That avoids kernel stalling behind slow + serial console and thus avoids softlockups, interrupt + timeouts, or userspace timing out during heavy printing. + However for debugging problems, printing messages to + console immediately may be desirable. This option + enables such behavior. + processor.max_cstate= [HW,ACPI] Limit processor to maximum C-state max_cstate=9 overrides any DMI blacklist limit. diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index d5fd844..323566b 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -46,6 +46,7 @@ #include <linux/utsname.h> #include <linux/ctype.h> #include <linux/uio.h> +#include <linux/kthread.h> #include <asm/uaccess.h> #include <asm-generic/sections.h> @@ -284,6 +285,98 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); static char *log_buf = __log_buf; static u32 log_buf_len = __LOG_BUF_LEN; +/* + * When true, printing to console will happen synchronously unless someone else + * is already printing messages. + * + * The default value on UP systems is 'true'. + */ +static bool __read_mostly printk_sync = !IS_ENABLED(CONFIG_SMP); +module_param_named(synchronous, printk_sync, bool, S_IRUGO | S_IWUSR); +MODULE_PARM_DESC(synchronous, "make printing to console synchronous"); + +/* Printing kthread for async vprintk_emit() */ +static struct task_struct *printk_thread; +/* Wait for printing wakeups from async vprintk_emit() */ +static DECLARE_WAIT_QUEUE_HEAD(printing_wait); + +static int printing_func(void *data) +{ + while (1) { + DECLARE_WAITQUEUE(wait, current); + set_current_state(TASK_INTERRUPTIBLE); + add_wait_queue(&printing_wait, &wait); + + schedule(); + remove_wait_queue(&printing_wait, &wait); + + console_lock(); + console_unlock(); + } + + return 0; +} + +static int __init init_printk_thread(void) +{ + if (printk_sync) + return 0; + + printk_thread = kthread_run(printing_func, NULL, "printk"); + BUG_ON(IS_ERR(printk_thread)); + return 0; +} +late_initcall(init_printk_thread); + +/* + * Delayed printk version, for scheduler-internal messages: + */ +#define PRINTK_PENDING_WAKEUP (1<<0) +#define PRINTK_PENDING_OUTPUT (1<<1) + +static DEFINE_PER_CPU(int, printk_pending); + +static void wake_up_klogd_work_func(struct irq_work *irq_work) +{ + int pending = __this_cpu_xchg(printk_pending, 0); + + if (pending & PRINTK_PENDING_OUTPUT) { + /* If trylock fails, someone else is doing the printing */ + if (console_trylock()) + console_unlock(); + } + + if (pending & PRINTK_PENDING_WAKEUP) + wake_up_interruptible(&log_wait); +} + +static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = { + .func = wake_up_klogd_work_func, + .flags = IRQ_WORK_LAZY, +}; + +void wake_up_klogd(void) +{ + preempt_disable(); + if (waitqueue_active(&log_wait)) { + this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP); + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); + } + preempt_enable(); +} + +int printk_deferred(const char *fmt, ...) +{ + va_list args; + int r; + + va_start(args, fmt); + r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args); + va_end(args); + + return r; +} + /* Return log buffer address */ char *log_buf_addr_get(void) { @@ -1609,6 +1702,8 @@ asmlinkage int vprintk_emit(int facility, int level, const char *dict, size_t dictlen, const char *fmt, va_list args) { + /* cpu currently holding logbuf_lock in this function */ + static unsigned int logbuf_cpu = UINT_MAX; static bool recursion_bug; static char textbuf[LOG_LINE_MAX]; char *text = textbuf; @@ -1618,12 +1713,17 @@ asmlinkage int vprintk_emit(int facility, int level, int this_cpu; int printed_len = 0; bool in_sched = false; - /* cpu currently holding logbuf_lock in this function */ - static unsigned int logbuf_cpu = UINT_MAX; + bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH; + bool sync_print = printk_sync; if (level == LOGLEVEL_SCHED) { level = LOGLEVEL_DEFAULT; + /* + * Deferred sched messages must not be printed + * synchronously regardless the @printk_sync or @in_panic. + */ in_sched = true; + sync_print = false; } boot_delay_msec(level); @@ -1752,10 +1852,41 @@ asmlinkage int vprintk_emit(int facility, int level, logbuf_cpu = UINT_MAX; raw_spin_unlock(&logbuf_lock); lockdep_on(); + /* + * By default we print message to console asynchronously so that kernel + * doesn't get stalled due to slow serial console. That can lead to + * softlockups, lost interrupts, or userspace timing out under heavy + * printing load. + * + * However we resort to synchronous printing of messages during early + * boot, when synchronous printing was explicitly requested by + * kernel parameter, or when console_verbose() was called to print + * everything during panic / oops. + */ + if (!sync_print) { + if (printk_thread && !in_panic) { + /* + * Wakeup the printing kthread and offload printing + * to a schedulable context. + */ + wake_up(&printing_wait); + } else if (in_sched) { + /* + * @in_sched messages may come too early, when we don't + * yet have @printk_thread. We can't print deferred + * messages directly, because this may deadlock, route + * them via IRQ context. + */ + __this_cpu_or(printk_pending, + PRINTK_PENDING_OUTPUT); + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); + } else { + sync_print = true; + } + } local_irq_restore(flags); - /* If called from the scheduler, we can not call up(). */ - if (!in_sched) { + if (sync_print) { lockdep_off(); /* * Try to acquire and then immediately release the console @@ -2724,60 +2855,6 @@ late_initcall(printk_late_init); #if defined CONFIG_PRINTK /* - * Delayed printk version, for scheduler-internal messages: - */ -#define PRINTK_PENDING_WAKEUP 0x01 -#define PRINTK_PENDING_OUTPUT 0x02 - -static DEFINE_PER_CPU(int, printk_pending); - -static void wake_up_klogd_work_func(struct irq_work *irq_work) -{ - int pending = __this_cpu_xchg(printk_pending, 0); - - if (pending & PRINTK_PENDING_OUTPUT) { - /* If trylock fails, someone else is doing the printing */ - if (console_trylock()) - console_unlock(); - } - - if (pending & PRINTK_PENDING_WAKEUP) - wake_up_interruptible(&log_wait); -} - -static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = { - .func = wake_up_klogd_work_func, - .flags = IRQ_WORK_LAZY, -}; - -void wake_up_klogd(void) -{ - preempt_disable(); - if (waitqueue_active(&log_wait)) { - this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP); - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); - } - preempt_enable(); -} - -int printk_deferred(const char *fmt, ...) -{ - va_list args; - int r; - - preempt_disable(); - va_start(args, fmt); - r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args); - va_end(args); - - __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT); - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); - preempt_enable(); - - return r; -} - -/* * printk rate limiting, lifted from the networking subsystem. * * This enforces a rate limit: not more than 10 kernel messages -- 2.8.0.rc0