irq: Track the interrupt timings

The interrupt framework gives a lot of information about each interrupt.

It does not keep track of when those interrupts occur though.

This patch provides a mean to record the timestamp for each interrupt
occurrences in a per-CPU circular buffer to help with the prediction
of the next occurrence using a statistical model.

Each CPU can store IRQ_TIMINGS_SIZE events <irq, timestamp>, the current
value of IRQ_TIMINGS_SIZE is 32.

A static key is introduced so when the irq prediction is switched off at
runtime, we can reduce the overhead near to zero. The irq timings is
supposed to be potentially used by different sub-systems and for this reason
the static key is a ref counter, so when the last use releases the irq
timings that will result on the effective deactivation of the irq measurement.

It results in most of the code in internals.h for inline reason and a very
few in the new file timings.c. The latter will contain more in the next patch
which will provide the statistical model for the next event prediction.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>

7 files changed, 110 insertions, 0 deletions

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index ad16809c8596..22dceab61129 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -664,6 +664,11 @@ static inline void init_irq_proc(void)
 }
 #endif
 
+#ifdef CONFIG_IRQ_TIMINGS
+void irq_timings_enable(void);
+void irq_timings_disable(void);
+#endif
+
 struct seq_file;
 int show_interrupts(struct seq_file *p, void *v);
 int arch_show_interrupts(struct seq_file *p, int prec);
diff --git a/kernel/irq/Kconfig b/kernel/irq/Kconfig
index 3b48dab80164..392c9f536663 100644
--- a/kernel/irq/Kconfig
+++ b/kernel/irq/Kconfig
@@ -77,6 +77,9 @@ config GENERIC_MSI_IRQ_DOMAIN
 config HANDLE_DOMAIN_IRQ
 	bool
 
+config IRQ_TIMINGS
+	bool
+
 config IRQ_DOMAIN_DEBUG
 	bool "Expose hardware/virtual IRQ mapping via debugfs"
 	depends on IRQ_DOMAIN && DEBUG_FS
diff --git a/kernel/irq/Makefile b/kernel/irq/Makefile
index 2fc9cbdf35b6..9c6d3e87fc94 100644
--- a/kernel/irq/Makefile
+++ b/kernel/irq/Makefile
@@ -8,3 +8,4 @@ obj-$(CONFIG_GENERIC_PENDING_IRQ) += migration.o
 obj-$(CONFIG_GENERIC_IRQ_MIGRATION) += cpuhotplug.o
 obj-$(CONFIG_PM_SLEEP) += pm.o
 obj-$(CONFIG_GENERIC_MSI_IRQ) += msi.o
+obj-$(CONFIG_IRQ_TIMINGS) += timings.o
diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index 57bff7857e87..12d8c31d07b0 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -138,6 +138,8 @@ irqreturn_t handle_irq_event_percpu(struct irq_desc *desc)
 	unsigned int flags = 0, irq = desc->irq_data.irq;
 	struct irqaction *action = desc->action;
 
+	record_irq_time(desc);
+
 	/* action might have become NULL since we dropped the lock */
 	while (action) {
 		irqreturn_t res;
diff --git a/kernel/irq/internals.h b/kernel/irq/internals.h
index fcab63c66905..02768ea34754 100644
--- a/kernel/irq/internals.h
+++ b/kernel/irq/internals.h
@@ -56,6 +56,7 @@ enum {
 	IRQS_WAITING		= 0x00000080,
 	IRQS_PENDING		= 0x00000200,
 	IRQS_SUSPENDED		= 0x00000800,
+	IRQS_TIMINGS		= 0x00001000,
 };
 
 #include "debug.h"
@@ -215,3 +216,68 @@ irq_pm_install_action(struct irq_desc *desc, struct irqaction *action) { }
 static inline void
 irq_pm_remove_action(struct irq_desc *desc, struct irqaction *action) { }
 #endif
+
+#ifdef CONFIG_IRQ_TIMINGS
+
+#define IRQ_TIMINGS_SHIFT	5
+#define IRQ_TIMINGS_SIZE	(1 << IRQ_TIMINGS_SHIFT)
+#define IRQ_TIMINGS_MASK	(IRQ_TIMINGS_SIZE - 1)
+
+struct irq_timing {
+	u32 irq;
+	u64 ts;
+};
+
+struct irq_timings {
+	struct irq_timing values[IRQ_TIMINGS_SIZE]; /* our circular buffer */
+	unsigned int count; /* Number of interruptions since last inspection */
+};
+
+DECLARE_PER_CPU(struct irq_timings, irq_timings);
+
+static inline void remove_timings(struct irq_desc *desc)
+{
+	desc->istate &= ~IRQS_TIMINGS;
+}
+
+static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
+{
+	/*
+	 * We don't need the measurement because the idle code already
+	 * knows the next expiry event.
+	 */
+	if (act->flags & __IRQF_TIMER)
+		return;
+
+	desc->istate |= IRQS_TIMINGS;
+}
+
+extern struct static_key_false irq_timing_enabled;
+
+/*
+ * The function record_irq_time is only called in one place in the
+ * interrupts handler. We want this function always inline so the code
+ * inside is embedded in the function and the static key branching
+ * code can act at the higher level. Without the explicit
+ * __always_inline we can end up with a function call and a small
+ * overhead in the hotpath for nothing.
+ */
+static __always_inline void record_irq_time(struct irq_desc *desc)
+{
+	if (static_key_enabled(&irq_timing_enabled)) {
+		if (desc->istate & IRQS_TIMINGS) {
+			struct irq_timings *timings = this_cpu_ptr(&irq_timings);
+			unsigned int index = timings->count & IRQ_TIMINGS_MASK;
+
+			timings->values[index].ts = local_clock();
+			timings->values[index].irq = irq_desc_get_irq(desc);
+			timings->count++;
+		}
+	}
+}
+#else
+static inline void remove_timings(struct irq_desc *desc) {}
+static inline void setup_timings(struct irq_desc *desc,
+				 struct irqaction *act) {};
+static inline void record_irq_time(struct irq_desc *desc) {}
+#endif /* CONFIG_IRQ_TIMINGS */
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 6ead200370da..786934353daf 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1345,6 +1345,8 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 		__enable_irq(desc);
 	}
 
+	setup_timings(desc, new);
+
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
 
 	/*
@@ -1467,6 +1469,7 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
 		irq_settings_clr_disable_unlazy(desc);
 		irq_shutdown(desc);
 		irq_release_resources(desc);
+		remove_timings(desc);
 	}
 
 #ifdef CONFIG_SMP
diff --git a/kernel/irq/timings.c b/kernel/irq/timings.c
new file mode 100644
index 000000000000..21ceca8f1258
--- /dev/null
+++ b/kernel/irq/timings.c
@@ -0,0 +1,30 @@
+/*
+ * linux/kernel/irq/timings.c
+ *
+ * Copyright (C) 2016, Linaro Ltd - Daniel Lezcano <daniel.lezcano@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+#include <linux/percpu.h>
+#include <linux/static_key.h>
+#include <linux/interrupt.h>
+#include <linux/irq.h>
+
+#include "internals.h"
+
+DEFINE_STATIC_KEY_FALSE(irq_timing_enabled);
+
+DEFINE_PER_CPU(struct irq_timings, irq_timings);
+
+void irq_timings_enable(void)
+{
+	static_branch_inc(&irq_timing_enabled);
+}
+
+void irq_timings_disable(void)
+{
+	static_branch_dec(&irq_timing_enabled);
+}