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Subject[PATCH RT 6/8] genirq: Allow disabling of softirq processing in irq thread context
From: Thomas Gleixner <tglx@linutronix.de>

The processing of softirqs in irq thread context is a performance gain
for the non-rt workloads of a system, but it's counterproductive for
interrupts which are explicitely related to the realtime
workload. Allow such interrupts to prevent softirq processing in their
thread context.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
include/linux/interrupt.h | 2 ++
include/linux/irq.h | 4 +++-
kernel/irq/manage.c | 13 ++++++++++++-
kernel/irq/settings.h | 12 ++++++++++++
kernel/softirq.c | 7 +++++++
5 files changed, 36 insertions(+), 2 deletions(-)
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 3142442..71c2c0b 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -61,6 +61,7 @@
* IRQF_NO_THREAD - Interrupt cannot be threaded
* IRQF_EARLY_RESUME - Resume IRQ early during syscore instead of at device
* resume time.
+ * IRQF_NO_SOFTIRQ_CALL - Do not process softirqs in the irq thread context (RT)
*/
#define IRQF_DISABLED 0x00000020
#define IRQF_SAMPLE_RANDOM 0x00000040
@@ -75,6 +76,7 @@
#define IRQF_FORCE_RESUME 0x00008000
#define IRQF_NO_THREAD 0x00010000
#define IRQF_EARLY_RESUME 0x00020000
+#define IRQF_NO_SOFTIRQ_CALL 0x00040000

#define IRQF_TIMER (__IRQF_TIMER | IRQF_NO_SUSPEND | IRQF_NO_THREAD)

diff --git a/include/linux/irq.h b/include/linux/irq.h
index baa397e..2df0acc 100644
--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -65,6 +65,7 @@ typedef void (*irq_preflow_handler_t)(struct irq_data *data);
* IRQ_NO_BALANCING - Interrupt cannot be balanced (affinity set)
* IRQ_MOVE_PCNTXT - Interrupt can be migrated from process context
* IRQ_NESTED_TRHEAD - Interrupt nests into another thread
+ * IRQ_NO_SOFTIRQ_CALL - No softirq processing in the irq thread context (RT)
*/
enum {
IRQ_TYPE_NONE = 0x00000000,
@@ -87,12 +88,13 @@ enum {
IRQ_MOVE_PCNTXT = (1 << 14),
IRQ_NESTED_THREAD = (1 << 15),
IRQ_NOTHREAD = (1 << 16),
+ IRQ_NO_SOFTIRQ_CALL = (1 << 18),
};

#define IRQF_MODIFY_MASK \
(IRQ_TYPE_SENSE_MASK | IRQ_NOPROBE | IRQ_NOREQUEST | \
IRQ_NOAUTOEN | IRQ_MOVE_PCNTXT | IRQ_LEVEL | IRQ_NO_BALANCING | \
- IRQ_PER_CPU | IRQ_NESTED_THREAD)
+ IRQ_PER_CPU | IRQ_NESTED_THREAD | IRQ_NO_SOFTIRQ_CALL)

#define IRQ_NO_BALANCING_MASK (IRQ_PER_CPU | IRQ_NO_BALANCING)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 3eb233e..cd98592 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -739,7 +739,15 @@ irq_forced_thread_fn(struct irq_desc *desc, struct irqaction *action)
local_bh_disable();
ret = action->thread_fn(action->irq, action->dev_id);
irq_finalize_oneshot(desc, action, false);
- local_bh_enable();
+ /*
+ * Interrupts which have real time requirements can be set up
+ * to avoid softirq processing in the thread handler. This is
+ * safe as these interrupts do not raise soft interrupts.
+ */
+ if (irq_settings_no_softirq_call(desc))
+ _local_bh_enable();
+ else
+ local_bh_enable();
return ret;
}

@@ -1031,6 +1039,9 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
irqd_set(&desc->irq_data, IRQD_NO_BALANCING);
}

+ if (new->flags & IRQF_NO_SOFTIRQ_CALL)
+ irq_settings_set_no_softirq_call(desc);
+
/* Set default affinity mask once everything is setup */
setup_affinity(irq, desc, mask);

diff --git a/kernel/irq/settings.h b/kernel/irq/settings.h
index f166783..e1f617f 100644
--- a/kernel/irq/settings.h
+++ b/kernel/irq/settings.h
@@ -13,6 +13,7 @@ enum {
_IRQ_MOVE_PCNTXT = IRQ_MOVE_PCNTXT,
_IRQ_NO_BALANCING = IRQ_NO_BALANCING,
_IRQ_NESTED_THREAD = IRQ_NESTED_THREAD,
+ _IRQ_NO_SOFTIRQ_CALL = IRQ_NO_SOFTIRQ_CALL,
_IRQF_MODIFY_MASK = IRQF_MODIFY_MASK,
};

@@ -24,6 +25,7 @@ enum {
#define IRQ_NOTHREAD GOT_YOU_MORON
#define IRQ_NOAUTOEN GOT_YOU_MORON
#define IRQ_NESTED_THREAD GOT_YOU_MORON
+#define IRQ_NO_SOFTIRQ_CALL GOT_YOU_MORON
#undef IRQF_MODIFY_MASK
#define IRQF_MODIFY_MASK GOT_YOU_MORON

@@ -34,6 +36,16 @@ irq_settings_clr_and_set(struct irq_desc *desc, u32 clr, u32 set)
desc->status_use_accessors |= (set & _IRQF_MODIFY_MASK);
}

+static inline bool irq_settings_no_softirq_call(struct irq_desc *desc)
+{
+ return desc->status_use_accessors & _IRQ_NO_SOFTIRQ_CALL;
+}
+
+static inline void irq_settings_set_no_softirq_call(struct irq_desc *desc)
+{
+ desc->status_use_accessors |= _IRQ_NO_SOFTIRQ_CALL;
+}
+
static inline bool irq_settings_is_per_cpu(struct irq_desc *desc)
{
return desc->status_use_accessors & _IRQ_PER_CPU;
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 3489d06..729cd0c 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -431,6 +431,13 @@ void local_bh_enable_ip(unsigned long ip)
}
EXPORT_SYMBOL(local_bh_enable_ip);

+void _local_bh_enable(void)
+{
+ current->softirq_nestcnt--;
+ migrate_enable();
+}
+EXPORT_SYMBOL(_local_bh_enable);
+
/* For tracing */
int notrace __in_softirq(void)
{
--
1.7.8.3



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