sched/core: Rework and clarify prepare_lock_switch()

The prepare_lock_switch() function has an unused parameter, and also the
function name was not descriptive. To improve readability and remove
the extra parameter, do the following changes:

* Move prepare_lock_switch() from kernel/sched/sched.h to
  kernel/sched/core.c, rename it to prepare_task(), and remove the
  unused parameter.

* Split the smp_store_release() out from finish_lock_switch() to a
  function named finish_task.

* Comments ajdustments.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171215140603.gxe5i2y6fg5ojfpp@smtp.gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 644fa2e..a794f81 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2045,7 +2045,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
         * If the owning (remote) CPU is still in the middle of schedule() with
         * this task as prev, wait until its done referencing the task.
         *
-        * Pairs with the smp_store_release() in finish_lock_switch().
+        * Pairs with the smp_store_release() in finish_task().
         *
         * This ensures that tasks getting woken will be fully ordered against
         * their previous state and preserve Program Order.
@@ -2571,6 +2571,50 @@ fire_sched_out_preempt_notifiers(struct task_struct *curr,
 
 #endif /* CONFIG_PREEMPT_NOTIFIERS */
 
+static inline void prepare_task(struct task_struct *next)
+{
+#ifdef CONFIG_SMP
+       /*
+        * Claim the task as running, we do this before switching to it
+        * such that any running task will have this set.
+        */
+       next->on_cpu = 1;
+#endif
+}
+
+static inline void finish_task(struct task_struct *prev)
+{
+#ifdef CONFIG_SMP
+       /*
+        * After ->on_cpu is cleared, the task can be moved to a different CPU.
+        * We must ensure this doesn't happen until the switch is completely
+        * finished.
+        *
+        * In particular, the load of prev->state in finish_task_switch() must
+        * happen before this.
+        *
+        * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
+        */
+       smp_store_release(&prev->on_cpu, 0);
+#endif
+}
+
+static inline void finish_lock_switch(struct rq *rq)
+{
+#ifdef CONFIG_DEBUG_SPINLOCK
+       /* this is a valid case when another task releases the spinlock */
+       rq->lock.owner = current;
+#endif
+       /*
+        * If we are tracking spinlock dependencies then we have to
+        * fix up the runqueue lock - which gets 'carried over' from
+        * prev into current:
+        */
+       spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
+
+       raw_spin_unlock_irq(&rq->lock);
+}
+
 /**
  * prepare_task_switch - prepare to switch tasks
  * @rq: the runqueue preparing to switch
@@ -2591,7 +2635,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
        sched_info_switch(rq, prev, next);
        perf_event_task_sched_out(prev, next);
        fire_sched_out_preempt_notifiers(prev, next);
-       prepare_lock_switch(rq, next);
+       prepare_task(next);
        prepare_arch_switch(next);
 }
 
@@ -2646,7 +2690,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
         * the scheduled task must drop that reference.
         *
         * We must observe prev->state before clearing prev->on_cpu (in
-        * finish_lock_switch), otherwise a concurrent wakeup can get prev
+        * finish_task), otherwise a concurrent wakeup can get prev
         * running on another CPU and we could rave with its RUNNING -> DEAD
         * transition, resulting in a double drop.
         */
@@ -2663,7 +2707,8 @@ static struct rq *finish_task_switch(struct task_struct *prev)
         * to use.
         */
        smp_mb__after_unlock_lock();
-       finish_lock_switch(rq, prev);
+       finish_task(prev);
+       finish_lock_switch(rq);
        finish_arch_post_lock_switch();
 
        fire_sched_in_preempt_notifiers(current);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b19552a..43f5d6e 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1328,47 +1328,6 @@ static inline int task_on_rq_migrating(struct task_struct *p)
 # define finish_arch_post_lock_switch()        do { } while (0)
 #endif
 
-static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
-{
-#ifdef CONFIG_SMP
-       /*
-        * We can optimise this out completely for !SMP, because the
-        * SMP rebalancing from interrupt is the only thing that cares
-        * here.
-        */
-       next->on_cpu = 1;
-#endif
-}
-
-static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
-{
-#ifdef CONFIG_SMP
-       /*
-        * After ->on_cpu is cleared, the task can be moved to a different CPU.
-        * We must ensure this doesn't happen until the switch is completely
-        * finished.
-        *
-        * In particular, the load of prev->state in finish_task_switch() must
-        * happen before this.
-        *
-        * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
-        */
-       smp_store_release(&prev->on_cpu, 0);
-#endif
-#ifdef CONFIG_DEBUG_SPINLOCK
-       /* this is a valid case when another task releases the spinlock */
-       rq->lock.owner = current;
-#endif
-       /*
-        * If we are tracking spinlock dependencies then we have to
-        * fix up the runqueue lock - which gets 'carried over' from
-        * prev into current:
-        */
-       spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
-
-       raw_spin_unlock_irq(&rq->lock);
-}
-
 /*
  * wake flags
  */