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Subject: [RFC 06/12] genirq: Add per-cpu flow handler with conditional IRQ stats
Date: Mon,  3 Jun 2024 22:09:34 -0700
Message-Id: <20240604050940.859909-7-mhklinux@outlook.com>
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From: Michael Kelley <mhklinux@outlook.com>

Hyper-V VMBus devices generate interrupts that are multiplexed
onto a single per-CPU architectural interrupt. The top-level VMBus
driver ISR demultiplexes these interrupts and invokes per-device
handlers. Currently, these per-device handlers are not modeled as
Linux IRQs, so /proc/interrupts shows all VMBus interrupts as accounted
to the top level architectural interrupt. Visibility into per-device
interrupt stats requires accessing VMBus-specific entries in sysfs.
The top-level VMBus driver ISR also handles management-related
interrupts that are not attributable to a particular VMBus device.

As part of changing VMBus to model VMBus per-device handlers as
normal Linux IRQs, the top-level VMBus driver needs to conditionally
account for interrupts. If it passes the interrupt off to a
device-specific IRQ, the interrupt stats are done by that IRQ
handler, and accounting for the interrupt at the top level
is duplicative. But if it handles a management-related interrupt
itself, then it should account for the interrupt itself.

Introduce a new flow handler that provides this functionality.
The new handler parallels handle_percpu_irq(), but does stats
only if the ISR returns other than IRQ_NONE. The existing
handle_untracked_irq() can't be used because it doesn't work for
per-cpu IRQs, and it doesn't provide conditional stats.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
 include/linux/irq.h |  1 +
 kernel/irq/chip.c   | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/include/linux/irq.h b/include/linux/irq.h
index a217e1029c1d..8825b95cefe5 100644
--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -656,6 +656,7 @@ extern void handle_edge_eoi_irq(struct irq_desc *desc);
 extern void handle_simple_irq(struct irq_desc *desc);
 extern void handle_untracked_irq(struct irq_desc *desc);
 extern void handle_percpu_irq(struct irq_desc *desc);
+extern void handle_percpu_demux_irq(struct irq_desc *desc);
 extern void handle_percpu_devid_irq(struct irq_desc *desc);
 extern void handle_bad_irq(struct irq_desc *desc);
 extern void handle_nested_irq(unsigned int irq);
diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
index dc94e0bf2c94..1f37a9db4a4d 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -910,6 +910,35 @@ void handle_percpu_irq(struct irq_desc *desc)
 		chip->irq_eoi(&desc->irq_data);
 }
 
+/**
+ *	handle_percpu_demux_irq - Per CPU local irq handler for IRQs
+ *	that may demultiplex to other IRQs
+ *	@desc:	the interrupt description structure for this irq
+ *
+ *	For per CPU interrupts on SMP machines without locking requirements.
+ *	Used for IRQs that may demultiplex to other IRQs that will do the
+ *     stats tracking and randomness. If the handler result indicates no
+ *	demultiplexing is done, account for the interrupt on this IRQ.
+ */
+void handle_percpu_demux_irq(struct irq_desc *desc)
+{
+	struct irq_chip *chip = irq_desc_get_chip(desc);
+
+	if (chip->irq_ack)
+		chip->irq_ack(&desc->irq_data);
+
+	if (__handle_irq_event_percpu(desc))
+		/*
+		 * PER CPU interrupts are not serialized.  Do not touch
+		 * desc->tot_count.
+		 */
+		__kstat_incr_irqs_this_cpu(desc);
+
+	if (chip->irq_eoi)
+		chip->irq_eoi(&desc->irq_data);
+}
+EXPORT_SYMBOL_GPL(handle_percpu_demux_irq);
+
 /**
  * handle_percpu_devid_irq - Per CPU local irq handler with per cpu dev ids
  * @desc:	the interrupt description structure for this irq
-- 
2.25.1