[PATCH 1/2] printk: Make printk() completely async

[Jan Kara]

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 by default printk() uses its own dedicated
workqueue with WQ_MEM_RECLAIM bit set. It falls back to system_long_wq
(console_unlock() is time unbound) only if it has failed to allocate
printk_wq. Another thing to mention, is that deferred printk() messages
may appear before printk_wq is allocated, so in the very beginning we
have to printk deferred messages the old way -- in IRQ context.

Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 Documentation/kernel-parameters.txt |  10 ++
 kernel/printk/printk.c              | 194 +++++++++++++++++++++++++-----------
 2 files changed, 146 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..d287fd7 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/workqueue.h>
 
 #include <asm/uaccess.h>
 #include <asm-generic/sections.h>
@@ -284,6 +285,108 @@ 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.
+ */
+static bool __read_mostly printk_sync;
+module_param_named(synchronous, printk_sync, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(synchronous, "make printing to console synchronous");
+
+static struct workqueue_struct *printk_wq;
+
+#define PRINTK_PENDING_WAKEUP	(1<<0)
+#define PRINTK_PENDING_IRQ_OUTPUT	(1<<1)
+#define PRINTK_PENDING_WQ_OUTPUT	(1<<2)
+
+static DEFINE_PER_CPU(int, printk_pending);
+
+static int __init init_printk_workqueue(void)
+{
+	if (printk_sync)
+		return 0;
+
+	printk_wq = alloc_workqueue("printk_wq", WQ_MEM_RECLAIM, 0);
+	/*
+	 * Fallback to one of system-wide workqueues if we failed to
+	 * allocate a dedicated one. console_unlock() may take some
+	 * time to flush all of the messages, so we better use
+	 * system_long_wq.
+	 */
+	if (!printk_wq) {
+		pr_err("printk: Fallback to system_long workqueue\n");
+		printk_wq = system_long_wq;
+	}
+
+	return 0;
+}
+late_initcall(init_printk_workqueue);
+
+static void printing_work_func(struct work_struct *work)
+{
+	console_lock();
+	console_unlock();
+}
+
+static DECLARE_WORK(printing_work, printing_work_func);
+
+static void wake_up_klogd_work_func(struct irq_work *irq_work)
+{
+	int pending = __this_cpu_xchg(printk_pending, 0);
+
+	/*
+	 * We just schedule regular work to do the printing from irq work. We
+	 * don't want to do printing here directly as that happens with
+	 * interrupts disabled and thus is bad for interrupt latency. We also
+	 * don't want to queue regular work from vprintk_emit() as that gets
+	 * called in various difficult contexts where schedule_work() could
+	 * deadlock.
+	 */
+	if (pending & PRINTK_PENDING_WQ_OUTPUT)
+		queue_work(printk_wq, &printing_work);
+
+	/*
+	 * Deferred printk messages may come too early when we don't have
+	 * workqueue allocated yet. We can't print deferred messages directly,
+	 * because this may deadlock, so in the very beginning we have to
+	 * print deferred messages from IRQ context.
+	 */
+	if (pending & PRINTK_PENDING_IRQ_OUTPUT) {
+		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 +1712,8 @@ 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 +1723,13 @@ 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;
 		in_sched = true;
+		sync_print = false;
 	}
 
 	boot_delay_msec(level);
@@ -1752,10 +1858,36 @@ 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.
+	 *
+	 * Deferred messages (in_sched), however, must not be printed
+	 * synchronously.
+	 */
+	if (!sync_print) {
+		if (printk_wq && !in_panic) {
+			__this_cpu_or(printk_pending,
+					PRINTK_PENDING_WQ_OUTPUT);
+			irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
+		} else if (in_sched) {
+			__this_cpu_or(printk_pending,
+					PRINTK_PENDING_IRQ_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 +2856,6 @@ 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.rc0.1.gd285ab0