[PATCH] Execute tasklets in the same order they were queued

[PATCH] Execute tasklets in the same order they were queued

[Olof Johansson]

I noticed this when looking at an openswan issue. Openswan (ab?)uses
the tasklet API to defer processing of packets in some situations,
with one packet per tasklet_action(). I started noticing sequences of
reverse-ordered sequence numbers coming over the wire, since new tasklets
are always queued at the head of the list but processed sequentially.

Convert it to instead append new entries to the tail of the list. As an
extra bonus, the splicing code in takeover_tasklets() no longer has to
iterate over the list.


Signed-off-by: Olof Johansson <olof@lixom.net>

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 5b3aea5..228c9c9 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -355,7 +355,8 @@ void open_softirq(int nr, void (*action)(struct softirq_action*), void *data)
 /* Tasklets */
 struct tasklet_head
 {
-	struct tasklet_struct *list;
+	struct tasklet_struct *head;
+	struct tasklet_struct **tail;
 };
 
 /* Some compilers disobey section attribute on statics when not
@@ -368,8 +369,9 @@ void __tasklet_schedule(struct tasklet_struct *t)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = __get_cpu_var(tasklet_vec).list;
-	__get_cpu_var(tasklet_vec).list = t;
+	t->next = NULL;
+	*__get_cpu_var(tasklet_vec).tail = t;
+	__get_cpu_var(tasklet_vec).tail = &(t->next);
 	raise_softirq_irqoff(TASKLET_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -381,8 +383,9 @@ void __tasklet_hi_schedule(struct tasklet_struct *t)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = __get_cpu_var(tasklet_hi_vec).list;
-	__get_cpu_var(tasklet_hi_vec).list = t;
+	t->next = NULL;
+	*__get_cpu_var(tasklet_hi_vec).tail = t;
+	__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
 	raise_softirq_irqoff(HI_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -394,8 +397,9 @@ static void tasklet_action(struct softirq_action *a)
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = __get_cpu_var(tasklet_vec).list;
-	__get_cpu_var(tasklet_vec).list = NULL;
+	list = __get_cpu_var(tasklet_vec).head;
+	__get_cpu_var(tasklet_vec).head = NULL;
+	__get_cpu_var(tasklet_vec).tail = &__get_cpu_var(tasklet_vec).head;
 	local_irq_enable();
 
 	while (list) {
@@ -415,8 +419,9 @@ static void tasklet_action(struct softirq_action *a)
 		}
 
 		local_irq_disable();
-		t->next = __get_cpu_var(tasklet_vec).list;
-		__get_cpu_var(tasklet_vec).list = t;
+		t->next = NULL;
+		*__get_cpu_var(tasklet_vec).tail = t;
+		__get_cpu_var(tasklet_vec).tail = &(t->next);
 		__raise_softirq_irqoff(TASKLET_SOFTIRQ);
 		local_irq_enable();
 	}
@@ -427,8 +432,9 @@ static void tasklet_hi_action(struct softirq_action *a)
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = __get_cpu_var(tasklet_hi_vec).list;
-	__get_cpu_var(tasklet_hi_vec).list = NULL;
+	list = __get_cpu_var(tasklet_hi_vec).head;
+	__get_cpu_var(tasklet_hi_vec).head = NULL;
+	__get_cpu_var(tasklet_hi_vec).tail = &__get_cpu_var(tasklet_hi_vec).head;
 	local_irq_enable();
 
 	while (list) {
@@ -448,8 +454,9 @@ static void tasklet_hi_action(struct softirq_action *a)
 		}
 
 		local_irq_disable();
-		t->next = __get_cpu_var(tasklet_hi_vec).list;
-		__get_cpu_var(tasklet_hi_vec).list = t;
+		t->next = NULL;
+		*__get_cpu_var(tasklet_hi_vec).tail = t;
+		__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
 		__raise_softirq_irqoff(HI_SOFTIRQ);
 		local_irq_enable();
 	}
@@ -486,6 +493,15 @@ EXPORT_SYMBOL(tasklet_kill);
 
 void __init softirq_init(void)
 {
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		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;
+	}
+
 	open_softirq(TASKLET_SOFTIRQ, tasklet_action, NULL);
 	open_softirq(HI_SOFTIRQ, tasklet_hi_action, NULL);
 }
@@ -554,9 +570,12 @@ void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 		return;
 
 	/* CPU is dead, so no lock needed. */
-	for (i = &per_cpu(tasklet_vec, cpu).list; *i; i = &(*i)->next) {
+	for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 		if (*i == t) {
 			*i = t->next;
+			/* If this was the tail element, move the tail ptr */
+			if (*i == NULL)
+				per_cpu(tasklet_vec, cpu).tail = i;
 			return;
 		}
 	}
@@ -571,14 +590,16 @@ static void takeover_tasklets(unsigned int cpu)
 	local_irq_disable();
 
 	/* Find end, append list for that CPU. */
-	for (i = &__get_cpu_var(tasklet_vec).list; *i; i = &(*i)->next);
-	*i = per_cpu(tasklet_vec, cpu).list;
-	per_cpu(tasklet_vec, cpu).list = NULL;
+	*__get_cpu_var(tasklet_vec).tail = per_cpu(tasklet_vec, cpu).head;
+	__get_cpu_var(tasklet_vec).tail = per_cpu(tasklet_vec, cpu).tail;
+	per_cpu(tasklet_vec, cpu).head = NULL;
+	per_cpu(tasklet_vec, cpu).next = &per_cpu(tasklet_vec, cpu).head;
 	raise_softirq_irqoff(TASKLET_SOFTIRQ);
 
-	for (i = &__get_cpu_var(tasklet_hi_vec).list; *i; i = &(*i)->next);
-	*i = per_cpu(tasklet_hi_vec, cpu).list;
-	per_cpu(tasklet_hi_vec, cpu).list = NULL;
+	*__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).head;
+	__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).tail;
+	per_cpu(tasklet_hi_vec, cpu).head = NULL;
+	per_cpu(tasklet_hi_vec, cpu).next = &per_cpu(tasklet_hi_vec, cpu).head;
 	raise_softirq_irqoff(HI_SOFTIRQ);
 
 	local_irq_enable();

Re: [PATCH] Execute tasklets in the same order they were queued

[Andrew Morton]

On Mon, 11 Feb 2008 16:28:13 -0600
Olof Johansson <olof@lixom.net> wrote:

> I noticed this when looking at an openswan issue. Openswan (ab?)uses
> the tasklet API to defer processing of packets in some situations,
> with one packet per tasklet_action(). I started noticing sequences of
> reverse-ordered sequence numbers coming over the wire, since new tasklets
> are always queued at the head of the list but processed sequentially.
> 
> Convert it to instead append new entries to the tail of the list. As an
> extra bonus, the splicing code in takeover_tasklets() no longer has to
> iterate over the list.
> 

hm, I'd have thought that this would already have caused problems in
networking.  And perhaps this change might have effects on networking too?

Probably it won't have _much_ effect on networking because networking
probably isn't queueing one tasklet per packet(!) but perhaps with bonded
channels or something like that?

Re: [PATCH] Execute tasklets in the same order they were queued

[Andrew Morton]

On Mon, 11 Feb 2008 16:28:13 -0600
Olof Johansson <olof@lixom.net> wrote:

> I noticed this when looking at an openswan issue. Openswan (ab?)uses
> the tasklet API to defer processing of packets in some situations,
> with one packet per tasklet_action(). I started noticing sequences of
> reverse-ordered sequence numbers coming over the wire, since new tasklets
> are always queued at the head of the list but processed sequentially.
> 
> Convert it to instead append new entries to the tail of the list. As an
> extra bonus, the splicing code in takeover_tasklets() no longer has to
> iterate over the list.

kernel/softirq.c: In function 'takeover_tasklets':
kernel/softirq.c:597: error: 'struct tasklet_head' has no member named 'next'
kernel/softirq.c:603: error: 'struct tasklet_head' has no member named 'next'
kernel/softirq.c:588: warning: unused variable 'i'

[PATCH v2] Execute tasklets in the same order they were queued

[Olof Johansson]

I noticed this when looking at an openswan issue. Openswan (ab?)uses
the tasklet API to defer processing of packets in some situations,
with one packet per tasklet_action(). I started noticing sequences
of backwards-ordered sequence numbers coming over the wire, since
new tasklets are always queued at the head of the list but processed
sequentially.

Convert it to instead append new entries to the tail of the list. As an
extra bonus, the splicing code in takeover_tasklets() no longer has to
iterate over the list.


Signed-off-by: Olof Johansson <olof@lixom.net>

---

On Tue, Feb 12, 2008 at 01:55:58PM -0800, Andrew Morton wrote:

> kernel/softirq.c: In function 'takeover_tasklets':
> kernel/softirq.c:597: error: 'struct tasklet_head' has no member named 'next'
> kernel/softirq.c:603: error: 'struct tasklet_head' has no member named 'next'
> kernel/softirq.c:588: warning: unused variable 'i'

How embarrassing. CONFIG_HOTPLUG_CPU was off when I built before I posted.

-Olof

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 5b3aea5..21fb1bd 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -355,7 +355,8 @@ void open_softirq(int nr, void (*action)(struct softirq_action*), void *data)
 /* Tasklets */
 struct tasklet_head
 {
-	struct tasklet_struct *list;
+	struct tasklet_struct *head;
+	struct tasklet_struct **tail;
 };
 
 /* Some compilers disobey section attribute on statics when not
@@ -368,8 +369,9 @@ void __tasklet_schedule(struct tasklet_struct *t)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = __get_cpu_var(tasklet_vec).list;
-	__get_cpu_var(tasklet_vec).list = t;
+	t->next = NULL;
+	*__get_cpu_var(tasklet_vec).tail = t;
+	__get_cpu_var(tasklet_vec).tail = &(t->next);
 	raise_softirq_irqoff(TASKLET_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -381,8 +383,9 @@ void __tasklet_hi_schedule(struct tasklet_struct *t)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = __get_cpu_var(tasklet_hi_vec).list;
-	__get_cpu_var(tasklet_hi_vec).list = t;
+	t->next = NULL;
+	*__get_cpu_var(tasklet_hi_vec).tail = t;
+	__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
 	raise_softirq_irqoff(HI_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -394,8 +397,9 @@ static void tasklet_action(struct softirq_action *a)
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = __get_cpu_var(tasklet_vec).list;
-	__get_cpu_var(tasklet_vec).list = NULL;
+	list = __get_cpu_var(tasklet_vec).head;
+	__get_cpu_var(tasklet_vec).head = NULL;
+	__get_cpu_var(tasklet_vec).tail = &__get_cpu_var(tasklet_vec).head;
 	local_irq_enable();
 
 	while (list) {
@@ -415,8 +419,9 @@ static void tasklet_action(struct softirq_action *a)
 		}
 
 		local_irq_disable();
-		t->next = __get_cpu_var(tasklet_vec).list;
-		__get_cpu_var(tasklet_vec).list = t;
+		t->next = NULL;
+		*__get_cpu_var(tasklet_vec).tail = t;
+		__get_cpu_var(tasklet_vec).tail = &(t->next);
 		__raise_softirq_irqoff(TASKLET_SOFTIRQ);
 		local_irq_enable();
 	}
@@ -427,8 +432,9 @@ static void tasklet_hi_action(struct softirq_action *a)
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = __get_cpu_var(tasklet_hi_vec).list;
-	__get_cpu_var(tasklet_hi_vec).list = NULL;
+	list = __get_cpu_var(tasklet_hi_vec).head;
+	__get_cpu_var(tasklet_hi_vec).head = NULL;
+	__get_cpu_var(tasklet_hi_vec).tail = &__get_cpu_var(tasklet_hi_vec).head;
 	local_irq_enable();
 
 	while (list) {
@@ -448,8 +454,9 @@ static void tasklet_hi_action(struct softirq_action *a)
 		}
 
 		local_irq_disable();
-		t->next = __get_cpu_var(tasklet_hi_vec).list;
-		__get_cpu_var(tasklet_hi_vec).list = t;
+		t->next = NULL;
+		*__get_cpu_var(tasklet_hi_vec).tail = t;
+		__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
 		__raise_softirq_irqoff(HI_SOFTIRQ);
 		local_irq_enable();
 	}
@@ -486,6 +493,15 @@ EXPORT_SYMBOL(tasklet_kill);
 
 void __init softirq_init(void)
 {
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		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;
+	}
+
 	open_softirq(TASKLET_SOFTIRQ, tasklet_action, NULL);
 	open_softirq(HI_SOFTIRQ, tasklet_hi_action, NULL);
 }
@@ -554,9 +570,12 @@ void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 		return;
 
 	/* CPU is dead, so no lock needed. */
-	for (i = &per_cpu(tasklet_vec, cpu).list; *i; i = &(*i)->next) {
+	for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 		if (*i == t) {
 			*i = t->next;
+			/* If this was the tail element, move the tail ptr */
+			if (*i == NULL)
+				per_cpu(tasklet_vec, cpu).tail = i;
 			return;
 		}
 	}
@@ -565,20 +584,20 @@ void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 
 static void takeover_tasklets(unsigned int cpu)
 {
-	struct tasklet_struct **i;
-
 	/* CPU is dead, so no lock needed. */
 	local_irq_disable();
 
 	/* Find end, append list for that CPU. */
-	for (i = &__get_cpu_var(tasklet_vec).list; *i; i = &(*i)->next);
-	*i = per_cpu(tasklet_vec, cpu).list;
-	per_cpu(tasklet_vec, cpu).list = NULL;
+	*__get_cpu_var(tasklet_vec).tail = per_cpu(tasklet_vec, cpu).head;
+	__get_cpu_var(tasklet_vec).tail = per_cpu(tasklet_vec, cpu).tail;
+	per_cpu(tasklet_vec, cpu).head = NULL;
+	per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 	raise_softirq_irqoff(TASKLET_SOFTIRQ);
 
-	for (i = &__get_cpu_var(tasklet_hi_vec).list; *i; i = &(*i)->next);
-	*i = per_cpu(tasklet_hi_vec, cpu).list;
-	per_cpu(tasklet_hi_vec, cpu).list = NULL;
+	*__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).head;
+	__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).tail;
+	per_cpu(tasklet_hi_vec, cpu).head = NULL;
+	per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 	raise_softirq_irqoff(HI_SOFTIRQ);
 
 	local_irq_enable();

Re: [PATCH v2] Execute tasklets in the same order they were queued

[Andrew Morton]

On Tue, 12 Feb 2008 16:06:47 -0600
Olof Johansson <olof@lixom.net> wrote:

> I noticed this when looking at an openswan issue. Openswan (ab?)uses
> the tasklet API to defer processing of packets in some situations,
> with one packet per tasklet_action(). I started noticing sequences
> of backwards-ordered sequence numbers coming over the wire, since
> new tasklets are always queued at the head of the list but processed
> sequentially.
> 
> Convert it to instead append new entries to the tail of the list. As an
> extra bonus, the splicing code in takeover_tasklets() no longer has to
> iterate over the list.

hm, well, let's see what if any effect this has on networking.

I'll cheerily tag this as to-be-merged-via-git-sched.  Ingo wasn't doing
much anyway.