linux-kernel - [PATCH 4/4] printk: Use unbound irq work for printing and waking

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Date:	Wed, 14 Aug 2013 15:28:28 +0200
From:	Jan Kara <jack@...e.cz>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>, mhocko@...e.cz,
	Jan Kara <jack@...e.cz>
Subject: [PATCH 4/4] printk: Use unbound irq work for printing and waking

Now when per-cpu printk buffers are gone, there's no need to have printk
flags or printk irq_work per cpu. Just make printk_pending a single
variable operated by atomic operations and have single unbound irq work
doing the waking and printing. This has an advantage that any cpu can do the
printing / wakeup work thus lowering the latency of printing and better
distributing the printing work over cpus.

Signed-off-by: Jan Kara <jack@...e.cz>
---
 kernel/printk/printk.c | 47 ++++++++++++++++++++++++++---------------------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 28c220a..118ba7b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2486,36 +2486,41 @@ late_initcall(printk_late_init);
 /*
  * Delayed printk version, for scheduler-internal messages:
  */
-#define PRINTK_PENDING_WAKEUP	0x01
-#define PRINTK_PENDING_OUTPUT	0x02
+#define PRINTK_PENDING_WAKEUP	0
+#define PRINTK_PENDING_OUTPUT	1
 
-static DEFINE_PER_CPU(int, printk_pending);
+static unsigned long printk_pending;
 
-static void wake_up_klogd_work_func(struct irq_work *irq_work)
+static void printk_irq_work_func(struct irq_work *irq_work)
 {
-	int pending = __this_cpu_xchg(printk_pending, 0);
+	if (printk_pending) {
+		unsigned long pending = xchg(&printk_pending, 0);
 
-	if (pending & PRINTK_PENDING_OUTPUT) {
-		/* If trylock fails, someone else is doing the printing */
-		if (console_trylock())
-			console_unlock();
-	}
+		if (test_bit(PRINTK_PENDING_OUTPUT, &pending)) {
+			/*
+			 * If trylock fails, someone else is doing the
+			 * printing
+			 */
+			if (console_trylock())
+				console_unlock();
+		}
 
-	if (pending & PRINTK_PENDING_WAKEUP)
-		wake_up_interruptible(&log_wait);
+		if (test_bit(PRINTK_PENDING_WAKEUP, &pending))
+			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,
+static struct irq_work printk_irq_work = {
+	.func = printk_irq_work_func,
+	.flags = IRQ_WORK_UNBOUND,
 };
 
 void wake_up_klogd(void)
 {
 	preempt_disable();
 	if (waitqueue_active(&log_wait)) {
-		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
-		irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
+		set_bit(PRINTK_PENDING_WAKEUP, &printk_pending);
+		irq_work_queue(&printk_irq_work);
 	}
 	preempt_enable();
 }
@@ -2529,8 +2534,8 @@ int printk_sched(const char *fmt, ...)
 	r = vprintk_emit(0, -2, NULL, 0, fmt, args);
 	va_end(args);
 
-	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
-	irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
+	set_bit(PRINTK_PENDING_OUTPUT, &printk_pending);
+	irq_work_queue(&printk_irq_work);
 
 	return r;
 }
@@ -2543,8 +2548,8 @@ void console_unlock(void)
 {
        if (__console_unlock()) {
                /* Leave the rest of printing for a timer tick */
-		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
-		irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
+		set_bit(PRINTK_PENDING_OUTPUT, &printk_pending);
+		irq_work_queue(&printk_irq_work);
 	}
 }
 EXPORT_SYMBOL(console_unlock);
-- 
1.8.1.4

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