[RFC PATCH 2/3] sched: add yield_to function

Rik van Riel
2010-12-02T19:44:23Z

Add a yield_to function to the scheduler code, allowing us to
give the remainder of our timeslice to another thread.

We may want to use this to provide a sys_yield_to system call
one day.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c5f926c..4f3cce9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1985,6 +1985,7 @@ extern void set_user_nice(struct task_struct *p, long nice);
extern int task_prio(const struct task_struct *p);
extern int task_nice(const struct task_struct *p);
extern int can_nice(const struct task_struct *p, const int nice);
+extern void requeue_task(struct rq *rq, struct task_struct *p);
extern int task_curr(const struct task_struct *p);
extern int idle_cpu(int cpu);
extern int sched_setscheduler(struct task_struct *, int, struct sched_param *);
@@ -2058,6 +2059,14 @@ extern int wake_up_state(struct task_struct *tsk, unsigned int state);
extern int wake_up_process(struct task_struct *tsk);
extern void wake_up_new_task(struct task_struct *tsk,
unsigned long clone_flags);
+
+#ifdef CONFIG_SCHED_HRTICK
+extern u64 slice_remain(struct task_struct *);
+extern void yield_to(struct task_struct *);
+#else
+static inline void yield_to(struct task_struct *p) yield()
+#endif
+
#ifdef CONFIG_SMP
extern void kick_process(struct task_struct *tsk);
#else
diff --git a/kernel/sched.c b/kernel/sched.c
index f8e5a25..ef088cd 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1909,6 +1909,26 @@ static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
p->se.on_rq = 0;
}

+/**
+ * requeue_task - requeue a task which priority got changed by yield_to
+ * @rq: the tasks's runqueue
+ * @p: the task in question
+ * Must be called with the runqueue lock held. Will cause the CPU to
+ * reschedule if p is now at the head of the runqueue.
+ */
+void requeue_task(struct rq *rq, struct task_struct *p)
+{
+ assert_spin_locked(&rq->lock);
+
+ if (!p->se.on_rq || task_running(rq, p) || task_has_rt_policy(p))
+ return;
+
+ dequeue_task(rq, p, 0);
+ enqueue_task(rq, p, 0);
+
+ resched_task(p);
+}
+
/*
* __normal_prio - return the priority that is based on the static prio
*/
@@ -6797,6 +6817,36 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
return ret;
}

+#ifdef CONFIG_SCHED_HRTICK
+/*
+ * Yield the CPU, giving the remainder of our time slice to task p.
+ * Typically used to hand CPU time to another thread inside the same
+ * process, eg. when p holds a resource other threads are waiting for.
+ * Giving priority to p may help get that resource released sooner.
+ */
+void yield_to(struct task_struct *p)
+{
+ unsigned long flags;
+ struct sched_entity *se = &p->se;
+ struct rq *rq;
+ struct cfs_rq *cfs_rq;
+ u64 remain = slice_remain(current);
+
+ rq = task_rq_lock(p, &flags);
+ if (task_running(rq, p) || task_has_rt_policy(p))
+ goto out;
+ cfs_rq = cfs_rq_of(se);
+ se->vruntime -= remain;
+ if (se->vruntime < cfs_rq->min_vruntime)
+ se->vruntime = cfs_rq->min_vruntime;
+ requeue_task(rq, p);
+ out:
+ task_rq_unlock(rq, &flags);
+ yield();
+}
+EXPORT_SYMBOL(yield_to);
+#endif
+
/**
* sys_sched_yield - yield the current processor to other threads.
*
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 5119b08..2a0a595 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -974,6 +974,25 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
*/

#ifdef CONFIG_SCHED_HRTICK
+u64 slice_remain(struct task_struct *p)
+{
+ unsigned long flags;
+ struct sched_entity *se = &p->se;
+ struct cfs_rq *cfs_rq;
+ struct rq *rq;
+ u64 slice, ran;
+ s64 delta;
+
+ rq = task_rq_lock(p, &flags);
+ cfs_rq = cfs_rq_of(se);
+ slice = sched_slice(cfs_rq, se);
+ ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
+ delta = slice - ran;
+ task_rq_unlock(rq, &flags);
+
+ return max(delta, 0LL);
+}
+
static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
{
struct sched_entity *se = &p->se;

--
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