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Subject[tip:sched/core] sched: Add yield_to(task, preempt) functionality
Commit-ID:  d95f412200652694e63e64bfd49f0ae274a54479
Gitweb: http://git.kernel.org/tip/d95f412200652694e63e64bfd49f0ae274a54479
Author: Mike Galbraith <efault@gmx.de>
AuthorDate: Tue, 1 Feb 2011 09:50:51 -0500
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Thu, 3 Feb 2011 14:20:33 +0100

sched: Add yield_to(task, preempt) functionality

Currently only implemented for fair class tasks.

Add a yield_to_task method() to the fair scheduling class. allowing the
caller of yield_to() to accelerate another thread in it's thread group,
task group.

Implemented via a scheduler hint, using cfs_rq->next to encourage the
target being selected. We can rely on pick_next_entity to keep things
fair, so noone can accelerate a thread that has already used its fair
share of CPU time.

This also means callers should only call yield_to when they really
mean it. Calling it too often can result in the scheduler just
ignoring the hint.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110201095051.4ddb7738@annuminas.surriel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/sched.h | 2 +
kernel/sched.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched_fair.c | 20 +++++++++++
3 files changed, 107 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4e9fad2..c88b3bf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1058,6 +1058,7 @@ struct sched_class {
void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
void (*yield_task) (struct rq *rq);
+ bool (*yield_to_task) (struct rq *rq, struct task_struct *p, bool preempt);

void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);

@@ -1972,6 +1973,7 @@ static inline int rt_mutex_getprio(struct task_struct *p)
# define rt_mutex_adjust_pi(p) do { } while (0)
#endif

+extern bool yield_to(struct task_struct *p, bool preempt);
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);
diff --git a/kernel/sched.c b/kernel/sched.c
index ae5e1a1..2effcb7 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1686,6 +1686,39 @@ static void double_rq_unlock(struct rq *rq1, struct rq *rq2)
__release(rq2->lock);
}

+#else /* CONFIG_SMP */
+
+/*
+ * double_rq_lock - safely lock two runqueues
+ *
+ * Note this does not disable interrupts like task_rq_lock,
+ * you need to do so manually before calling.
+ */
+static void double_rq_lock(struct rq *rq1, struct rq *rq2)
+ __acquires(rq1->lock)
+ __acquires(rq2->lock)
+{
+ BUG_ON(!irqs_disabled());
+ BUG_ON(rq1 != rq2);
+ raw_spin_lock(&rq1->lock);
+ __acquire(rq2->lock); /* Fake it out ;) */
+}
+
+/*
+ * double_rq_unlock - safely unlock two runqueues
+ *
+ * Note this does not restore interrupts like task_rq_unlock,
+ * you need to do so manually after calling.
+ */
+static void double_rq_unlock(struct rq *rq1, struct rq *rq2)
+ __releases(rq1->lock)
+ __releases(rq2->lock)
+{
+ BUG_ON(rq1 != rq2);
+ raw_spin_unlock(&rq1->lock);
+ __release(rq2->lock);
+}
+
#endif

static void calc_load_account_idle(struct rq *this_rq);
@@ -5448,6 +5481,58 @@ void __sched yield(void)
}
EXPORT_SYMBOL(yield);

+/**
+ * yield_to - yield the current processor to another thread in
+ * your thread group, or accelerate that thread toward the
+ * processor it's on.
+ *
+ * It's the caller's job to ensure that the target task struct
+ * can't go away on us before we can do any checks.
+ *
+ * Returns true if we indeed boosted the target task.
+ */
+bool __sched yield_to(struct task_struct *p, bool preempt)
+{
+ struct task_struct *curr = current;
+ struct rq *rq, *p_rq;
+ unsigned long flags;
+ bool yielded = 0;
+
+ local_irq_save(flags);
+ rq = this_rq();
+
+again:
+ p_rq = task_rq(p);
+ double_rq_lock(rq, p_rq);
+ while (task_rq(p) != p_rq) {
+ double_rq_unlock(rq, p_rq);
+ goto again;
+ }
+
+ if (!curr->sched_class->yield_to_task)
+ goto out;
+
+ if (curr->sched_class != p->sched_class)
+ goto out;
+
+ if (task_running(p_rq, p) || p->state)
+ goto out;
+
+ yielded = curr->sched_class->yield_to_task(rq, p, preempt);
+ if (yielded)
+ schedstat_inc(rq, yld_count);
+
+out:
+ double_rq_unlock(rq, p_rq);
+ local_irq_restore(flags);
+
+ if (yielded)
+ schedule();
+
+ return yielded;
+}
+EXPORT_SYMBOL_GPL(yield_to);
+
/*
* This task is about to go to sleep on IO. Increment rq->nr_iowait so
* that process accounting knows that this is a task in IO wait state.
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index c0fbeb9..0270246 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1975,6 +1975,25 @@ static void yield_task_fair(struct rq *rq)
set_skip_buddy(se);
}

+static bool yield_to_task_fair(struct rq *rq, struct task_struct *p, bool preempt)
+{
+ struct sched_entity *se = &p->se;
+
+ if (!se->on_rq)
+ return false;
+
+ /* Tell the scheduler that we'd really like pse to run next. */
+ set_next_buddy(se);
+
+ /* Make p's CPU reschedule; pick_next_entity takes care of fairness. */
+ if (preempt)
+ resched_task(rq->curr);
+
+ yield_task_fair(rq);
+
+ return true;
+}
+
#ifdef CONFIG_SMP
/**************************************************
* Fair scheduling class load-balancing methods:
@@ -4243,6 +4262,7 @@ static const struct sched_class fair_sched_class = {
.enqueue_task = enqueue_task_fair,
.dequeue_task = dequeue_task_fair,
.yield_task = yield_task_fair,
+ .yield_to_task = yield_to_task_fair,

.check_preempt_curr = check_preempt_wakeup,


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