[PATCH] sched: arch preempt notifier mechanism

This adds a general mechanism whereby a task can request the scheduler to
notify it whenever it is preempted or scheduled back in.  This allows the
task to swap any special-purpose registers like the fpu or Intel's VT
registers.

Signed-off-by: Avi Kivity <avi@qumranet.com>
[ mingo@elte.hu: fixes, cleanups ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 74 insertions, 2 deletions

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index c64ce9c14207..6b066632e40c 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -63,3 +63,6 @@ config PREEMPT_BKL
           Say Y here if you are building a kernel for a desktop system.
           Say N if you are unsure.
 
+config PREEMPT_NOTIFIERS
+        bool
+
diff --git a/kernel/sched.c b/kernel/sched.c
index 93cf241cfbe9..e901aa59f206 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1592,6 +1592,10 @@ static void __sched_fork(struct task_struct *p)
         INIT_LIST_HEAD(&p->run_list);
         p->se.on_rq = 0;
 
+#ifdef CONFIG_PREEMPT_NOTIFIERS
+        INIT_HLIST_HEAD(&p->preempt_notifiers);
+#endif
+
         /*
          * We mark the process as running here, but have not actually
          * inserted it onto the runqueue yet. This guarantees that
@@ -1673,6 +1677,63 @@ void fastcall wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
         task_rq_unlock(rq, &flags);
 }
 
+#ifdef CONFIG_PREEMPT_NOTIFIERS
+
+/**
+ * preempt_notifier_register - tell me when current is being being preempted
+ *                         and rescheduled
+ */
+void preempt_notifier_register(struct preempt_notifier *notifier)
+{
+        hlist_add_head(&notifier->link, &current->preempt_notifiers);
+}
+EXPORT_SYMBOL_GPL(preempt_notifier_register);
+
+/**
+ * preempt_notifier_unregister - no longer interested in preemption notifications
+ *
+ * This is safe to call from within a preemption notifier.
+ */
+void preempt_notifier_unregister(struct preempt_notifier *notifier)
+{
+        hlist_del(&notifier->link);
+}
+EXPORT_SYMBOL_GPL(preempt_notifier_unregister);
+
+static void fire_sched_in_preempt_notifiers(struct task_struct *curr)
+{
+        struct preempt_notifier *notifier;
+        struct hlist_node *node;
+
+        hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
+                notifier->ops->sched_in(notifier, raw_smp_processor_id());
+}
+
+static void
+fire_sched_out_preempt_notifiers(struct task_struct *curr,
+                                 struct task_struct *next)
+{
+        struct preempt_notifier *notifier;
+        struct hlist_node *node;
+
+        hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
+                notifier->ops->sched_out(notifier, next);
+}
+
+#else
+
+static void fire_sched_in_preempt_notifiers(struct task_struct *curr)
+{
+}
+
+static void
+fire_sched_out_preempt_notifiers(struct task_struct *curr,
+                                 struct task_struct *next)
+{
+}
+
+#endif
+
 /**
  * prepare_task_switch - prepare to switch tasks
  * @rq: the runqueue preparing to switch
@@ -1685,8 +1746,11 @@ void fastcall wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
  * prepare_task_switch sets up locking and calls architecture specific
  * hooks.
  */
-static inline void prepare_task_switch(struct rq *rq, struct task_struct *next)
+static inline void
+prepare_task_switch(struct rq *rq, struct task_struct *prev,
+                    struct task_struct *next)
 {
+        fire_sched_out_preempt_notifiers(prev, next);
         prepare_lock_switch(rq, next);
         prepare_arch_switch(next);
 }
@@ -1728,6 +1792,7 @@ static inline void finish_task_switch(struct rq *rq, struct task_struct *prev)
         prev_state = prev->state;
         finish_arch_switch(prev);
         finish_lock_switch(rq, prev);
+        fire_sched_in_preempt_notifiers(current);
         if (mm)
                 mmdrop(mm);
         if (unlikely(prev_state == TASK_DEAD)) {
@@ -1768,7 +1833,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 {
         struct mm_struct *mm, *oldmm;
 
-        prepare_task_switch(rq, next);
+        prepare_task_switch(rq, prev, next);
         mm = next->mm;
         oldmm = prev->active_mm;
         /*
@@ -6335,6 +6400,10 @@ void __init sched_init(void)
 
         set_load_weight(&init_task);
 
+#ifdef CONFIG_PREEMPT_NOTIFIERS
+        INIT_HLIST_HEAD(&init_task.preempt_notifiers);
+#endif
+
 #ifdef CONFIG_SMP
         nr_cpu_ids = highest_cpu + 1;
         open_softirq(SCHED_SOFTIRQ, run_rebalance_domains, NULL);