Patchwork [2/2] : softirq: Add support for triggering softirq work on softirqs.

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Submitter David Miller
Date Sept. 20, 2008, 10:56 a.m.
Message ID <20080920.035602.226564809.davem@davemloft.net>
Download mbox | patch
Permalink /patch/709/
State RFC
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Comments

David Miller - Sept. 20, 2008, 10:56 a.m.
From: Andrew Morton <akpm@linux-foundation.org>
Date: Sat, 20 Sep 2008 00:46:08 -0700

> Took a little staring to work out what that test is doing.  Adding
> 
> 	/* If the list was previouly empty, trigger a softirq run */
> 
> would be nice.

Ok.

> CONFIG_USE_GENERIC_SMP_HELPERS=y, CONFIG_SMP=n shouldn't be possible,
> if we care..

Sure.

> OK, now what's going on with call_single_data.flags?

I've split flags into "priv" and "flags" u16's.

> Is locking needed to keep that CPU online after this test?

You already know :)

> Ah.  So it is required that the __send_remote_softirq() caller disable
> local interrupts.  There's my locking.  It's worth a mention in the
> interface description, if not a WARN_ON().

Fair enough.

> It's best to provide some documentation for global, exported-to-modules
> functions please.

I've got your kerneldoc right here buddy boy...

> Could use __local_trigger() here I think, although that wouldn't really
> improve anything much.

This is a splice, so we'd need to iterate which would be cumbersome
and error prone.

Here is the updated patch.

softirq: Add support for triggering softirq work on softirqs.

This is basically a genericization of Jens Axboe's block layer
remote softirq changes.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 include/linux/interrupt.h |   13 +++++
 include/linux/smp.h       |    4 +-
 kernel/softirq.c          |  128 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 144 insertions(+), 1 deletions(-)
Andrew Morton - Sept. 20, 2008, 5:42 p.m.
On Sat, 20 Sep 2008 03:56:02 -0700 (PDT) David Miller <davem@davemloft.net> wrote:

> I've got your kerneldoc right here buddy boy...

thanks.  It's surprising how much that improves the code review.

> +/***

/** is sufficient & conventional for kerneldoc.

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Patch

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index fdd7b90..d8713c7 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -11,6 +11,8 @@ 
 #include <linux/hardirq.h>
 #include <linux/sched.h>
 #include <linux/irqflags.h>
+#include <linux/smp.h>
+#include <linux/percpu.h>
 #include <asm/atomic.h>
 #include <asm/ptrace.h>
 #include <asm/system.h>
@@ -271,7 +273,18 @@  extern void softirq_init(void);
 #define __raise_softirq_irqoff(nr) do { or_softirq_pending(1UL << (nr)); } while (0)
 extern void raise_softirq_irqoff(unsigned int nr);
 extern void raise_softirq(unsigned int nr);
+DECLARE_PER_CPU(struct list_head, softirq_work_list[NR_SOFTIRQ]);
 
+/* Try to send a softirq to a remote cpu.  If this cannot be done, the
+ * work will be queued to the local cpu.
+ */
+extern void send_remote_softirq(struct call_single_data *cp, int cpu, int softirq);
+
+/* Like send_remote_softirq(), but the caller must disable local cpu interrupts
+ * and compute the current cpu, passed in as 'this_cpu'.
+ */
+extern void __send_remote_softirq(struct call_single_data *cp, int cpu,
+				  int this_cpu, int softirq);
 
 /* Tasklets --- multithreaded analogue of BHs.
 
diff --git a/include/linux/smp.h b/include/linux/smp.h
index 66484d4..2e4d58b 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -7,6 +7,7 @@ 
  */
 
 #include <linux/errno.h>
+#include <linux/types.h>
 #include <linux/list.h>
 #include <linux/cpumask.h>
 
@@ -16,7 +17,8 @@  struct call_single_data {
 	struct list_head list;
 	void (*func) (void *info);
 	void *info;
-	unsigned int flags;
+	u16 flags;
+	u16 priv;
 };
 
 #ifdef CONFIG_SMP
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 27642a2..6c7b226 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -6,6 +6,8 @@ 
  *	Distribute under GPLv2.
  *
  *	Rewritten. Old one was good in 2.2, but in 2.3 it was immoral. --ANK (990903)
+ *
+ *	Remote softirq infrastructure is by Jens Axboe.
  */
 
 #include <linux/module.h>
@@ -463,17 +465,143 @@  void tasklet_kill(struct tasklet_struct *t)
 
 EXPORT_SYMBOL(tasklet_kill);
 
+DEFINE_PER_CPU(struct list_head, softirq_work_list[NR_SOFTIRQ]);
+
+static void __local_trigger(struct call_single_data *cp, int softirq)
+{
+	struct list_head *head = &__get_cpu_var(softirq_work_list[softirq]);
+
+	list_add_tail(&cp->list, head);
+
+	/* Trigger the softirq only if the list was previously empty.  */
+	if (head->next == &cp->list)
+		raise_softirq_irqoff(softirq);
+}
+
+#ifdef CONFIG_USE_GENERIC_SMP_HELPERS
+static void remote_softirq_receive(void *data)
+{
+	struct call_single_data *cp = data;
+	unsigned long flags;
+	int softirq;
+
+	softirq = cp->priv;
+
+	local_irq_save(flags);
+	__local_trigger(cp, softirq);
+	local_irq_restore(flags);
+}
+
+static int __try_remote_softirq(struct call_single_data *cp, int cpu, int softirq)
+{
+	if (cpu_online(cpu)) {
+		cp->func = remote_softirq_receive;
+		cp->info = cp;
+		cp->flags = 0;
+		cp->priv = softirq;
+
+		__smp_call_function_single(cpu, cp);
+		return 0;
+	}
+	return 1;
+}
+#else /* CONFIG_USE_GENERIC_SMP_HELPERS */
+static int __try_remote_softirq(struct call_single_data *cp, int cpu, int softirq)
+{
+	return 1;
+}
+#endif
+
+/***
+ * __send_remote_softirq - try to schedule softirq work on a remote cpu
+ * @cp: private SMP call function data area
+ * @cpu: the remote cpu
+ * @this_cpu: the currently executing cpu
+ * @softirq: the softirq for the work
+ *
+ * Attempt to schedule softirq work on a remote cpu.  If this cannot be
+ * done, the work is instead queued up on the local cpu.
+ *
+ * Interrupts must be disabled.
+ */
+void __send_remote_softirq(struct call_single_data *cp, int cpu, int this_cpu, int softirq)
+{
+	if (cpu == this_cpu || __try_remote_softirq(cp, cpu, softirq))
+		__local_trigger(cp, softirq);
+}
+EXPORT_SYMBOL(__send_remote_softirq);
+
+/***
+ * send_remote_softirq - try to schedule softirq work on a remote cpu
+ * @cp: private SMP call function data area
+ * @cpu: the remote cpu
+ * @softirq: the softirq for the work
+ *
+ * Like __send_remote_softirq except that disabling interrupts and
+ * computing the current cpu is done for the caller.
+ */
+void send_remote_softirq(struct call_single_data *cp, int cpu, int softirq)
+{
+	unsigned long flags;
+	int this_cpu;
+
+	local_irq_save(flags);
+	this_cpu = smp_processor_id();
+	__send_remote_softirq(cp, cpu, this_cpu, softirq);
+	local_irq_restore(flags);
+}
+EXPORT_SYMBOL(send_remote_softirq);
+
+static int __cpuinit remote_softirq_cpu_notify(struct notifier_block *self,
+					       unsigned long action, void *hcpu)
+{
+	/*
+	 * If a CPU goes away, splice its entries to the current CPU
+	 * and trigger a run of the softirq
+	 */
+	if (action == CPU_DEAD || action == CPU_DEAD_FROZEN) {
+		int cpu = (unsigned long) hcpu;
+		int i;
+
+		local_irq_disable();
+		for (i = 0; i < NR_SOFTIRQ; i++) {
+			struct list_head *head = &per_cpu(softirq_work_list[i], cpu);
+			struct list_head *local_head;
+
+			if (list_empty(head))
+				continue;
+
+			local_head = &__get_cpu_var(softirq_work_list[i]);
+			list_splice_init(head, local_head);
+			raise_softirq_irqoff(i);
+		}
+		local_irq_enable();
+	}
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata remote_softirq_cpu_notifier = {
+	.notifier_call	= remote_softirq_cpu_notify,
+};
+
 void __init softirq_init(void)
 {
 	int cpu;
 
 	for_each_possible_cpu(cpu) {
+		int i;
+
 		per_cpu(tasklet_vec, cpu).tail =
 			&per_cpu(tasklet_vec, cpu).head;
 		per_cpu(tasklet_hi_vec, cpu).tail =
 			&per_cpu(tasklet_hi_vec, cpu).head;
+		for (i = 0; i < NR_SOFTIRQ; i++)
+			INIT_LIST_HEAD(&per_cpu(softirq_work_list[i], cpu));
 	}
 
+	register_hotcpu_notifier(&remote_softirq_cpu_notifier);
+
 	open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 	open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 }