patches/sched-mmdrop-delayed.patch

Subject: sched: Move mmdrop to RCU on RT
From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 06 Jun 2011 12:20:33 +0200

Takes sleeping locks and calls into the memory allocator, so nothing
we want to do in task switch and oder atomic contexts.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/mm_types.h |    4 ++++
 include/linux/sched/mm.h |   11 +++++++++++
 kernel/fork.c            |   13 +++++++++++++
 kernel/sched/core.c      |   19 +++++++++++++++++--
 4 files changed, 45 insertions(+), 2 deletions(-)

--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -12,6 +12,7 @@
 #include <linux/completion.h>
 #include <linux/cpumask.h>
 #include <linux/uprobes.h>
+#include <linux/rcupdate.h>
 #include <linux/page-flags-layout.h>
 #include <linux/workqueue.h>
 
@@ -491,6 +492,9 @@ struct mm_struct {
 	bool tlb_flush_batched;
 #endif
 	struct uprobes_state uprobes_state;
+#ifdef CONFIG_PREEMPT_RT_BASE
+	struct rcu_head delayed_drop;
+#endif
 #ifdef CONFIG_HUGETLB_PAGE
 	atomic_long_t hugetlb_usage;
 #endif
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -49,6 +49,17 @@ static inline void mmdrop(struct mm_stru
 		__mmdrop(mm);
 }
 
+#ifdef CONFIG_PREEMPT_RT_BASE
+extern void __mmdrop_delayed(struct rcu_head *rhp);
+static inline void mmdrop_delayed(struct mm_struct *mm)
+{
+	if (atomic_dec_and_test(&mm->mm_count))
+		call_rcu(&mm->delayed_drop, __mmdrop_delayed);
+}
+#else
+# define mmdrop_delayed(mm)	mmdrop(mm)
+#endif
+
 /**
  * mmget() - Pin the address space associated with a &struct mm_struct.
  * @mm: The address space to pin.
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -605,6 +605,19 @@ void __mmdrop(struct mm_struct *mm)
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
 
+#ifdef CONFIG_PREEMPT_RT_BASE
+/*
+ * RCU callback for delayed mm drop. Not strictly rcu, but we don't
+ * want another facility to make this work.
+ */
+void __mmdrop_delayed(struct rcu_head *rhp)
+{
+	struct mm_struct *mm = container_of(rhp, struct mm_struct, delayed_drop);
+
+	__mmdrop(mm);
+}
+#endif
+
 static void mmdrop_async_fn(struct work_struct *work)
 {
 	struct mm_struct *mm;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2769,9 +2769,13 @@ static struct rq *finish_task_switch(str
 	 *   provided by mmdrop(),
 	 * - a sync_core for SYNC_CORE.
 	 */
+	/*
+	 * We use mmdrop_delayed() here so we don't have to do the
+	 * full __mmdrop() when we are the last user.
+	 */
 	if (mm) {
 		membarrier_mm_sync_core_before_usermode(mm);
-		mmdrop(mm);
+		mmdrop_delayed(mm);
 	}
 	if (unlikely(prev_state & (TASK_DEAD|TASK_PARKED))) {
 		switch (prev_state) {
@@ -5528,6 +5532,8 @@ void sched_setnuma(struct task_struct *p
 #endif /* CONFIG_NUMA_BALANCING */
 
 #ifdef CONFIG_HOTPLUG_CPU
+static DEFINE_PER_CPU(struct mm_struct *, idle_last_mm);
+
 /*
  * Ensure that the idle task is using init_mm right before its CPU goes
  * offline.
@@ -5542,7 +5548,12 @@ void idle_task_exit(void)
 		switch_mm(mm, &init_mm, current);
 		finish_arch_post_lock_switch();
 	}
-	mmdrop(mm);
+	/*
+	 * Defer the cleanup to an alive cpu. On RT we can neither
+	 * call mmdrop() nor mmdrop_delayed() from here.
+	 */
+	per_cpu(idle_last_mm, smp_processor_id()) = mm;
+
 }
 
 /*
@@ -5845,6 +5856,10 @@ int sched_cpu_dying(unsigned int cpu)
 	update_max_interval();
 	nohz_balance_exit_idle(cpu);
 	hrtick_clear(rq);
+	if (per_cpu(idle_last_mm, cpu)) {
+		mmdrop_delayed(per_cpu(idle_last_mm, cpu));
+		per_cpu(idle_last_mm, cpu) = NULL;
+	}
 	return 0;
 }
 #endif