Assuming units of PREEMPT_DISABLE_OFFSET for preempt_count() numbers.

Now that TASK_DEAD no longer results in preempt_count() == 3 during
scheduling, we will always call context_switch() with preempt_count()
== 2.

However, we don't always end up with preempt_count() == 2 in
finish_task_switch() because new tasks get created with
preempt_count() == 1.

Create FORK_PREEMPT_COUNT and set it to 2 and use that in the right
places. Note that we cannot use INIT_PREEMPT_COUNT as that serves
another purpose (boot).

After this, preempt_count() is invariant across the context switch,
with exception of PREEMPT_ACTIVE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/preempt.h |    2 +-
 include/asm-generic/preempt.h  |    2 +-
 include/linux/sched.h          |   17 ++++++++++++-----
 kernel/sched/core.c            |   23 +++++++++++++++++++++--
 4 files changed, 35 insertions(+), 9 deletions(-)

--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -31,7 +31,7 @@ static __always_inline void preempt_coun
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->saved_preempt_count = FORK_PREEMPT_COUNT; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -24,7 +24,7 @@ static __always_inline void preempt_coun
  * must be macros to avoid header recursion hell
  */
 #define init_task_preempt_count(p) do { \
-	task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
+	task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \
 } while (0)
 
 #define init_idle_preempt_count(p, cpu) do { \
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -599,11 +599,7 @@ struct task_cputime_atomic {
 		.sum_exec_runtime = ATOMIC64_INIT(0),		\
 	}
 
-#ifdef CONFIG_PREEMPT_COUNT
-#define PREEMPT_DISABLED	(1 + PREEMPT_ENABLED)
-#else
-#define PREEMPT_DISABLED	PREEMPT_ENABLED
-#endif
+#define PREEMPT_DISABLED	(PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
 
 /*
  * Disable preemption until the scheduler is running -- use an unconditional
@@ -613,6 +609,17 @@ struct task_cputime_atomic {
  */
 #define INIT_PREEMPT_COUNT	PREEMPT_OFFSET
 
+/*
+ * Initial preempt_count value; reflects the preempt_count schedule invariant
+ * which states that during context switches:
+ *
+ *    preempt_count() == 2*PREEMPT_DISABLE_OFFSET
+ *
+ * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
+ * Note: See finish_task_switch().
+ */
+#define FORK_PREEMPT_COUNT	(2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
+
 /**
  * struct thread_group_cputimer - thread group interval timer counts
  * @cputime_atomic:	atomic thread group interval timers.
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2504,6 +2504,18 @@ static struct rq *finish_task_switch(str
 	struct mm_struct *mm = rq->prev_mm;
 	long prev_state;
 
+	/*
+	 * The previous task will have left us with a preempt_count of 2
+	 * because it left us after:
+	 *
+	 *	schedule()
+	 *	  preempt_disable();			// 1
+	 *	  __schedule()
+	 *	    raw_spin_lock_irq(&rq->lock)	// 2
+	 *
+	 * Also, see FORK_PREEMPT_COUNT.
+	 */
+
 	rq->prev_mm = NULL;
 
 	/*
@@ -2588,8 +2600,15 @@ asmlinkage __visible void schedule_tail(
 {
 	struct rq *rq;
 
-	/* finish_task_switch() drops rq->lock and enables preemtion */
-	preempt_disable();
+	/*
+	 * New tasks start with FORK_PREEMPT_COUNT, see there and
+	 * finish_task_switch() for details.
+	 *
+	 * finish_task_switch() will drop rq->lock() and lower preempt_count
+	 * and the preempt_enable() will end up enabling preemption (on
+	 * PREEMPT_COUNT kernels).
+	 */
+
 	rq = finish_task_switch(prev);
 	balance_callback(rq);
 	preempt_enable();


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