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Subject[PATCH 13/22] sched: Add p->pi_lock to task_rq_lock()
In order to be able to call set_task_cpu() while either holding
p->pi_lock or task_rq(p)->lock we need to hold both locks in order to
stabilize task_rq().

This makes task_rq_lock() acquire both locks, and have
__task_rq_lock() validate that p->pi_lock is held. This increases the
locking overhead for most scheduler syscalls but allows reduction of
rq->lock contention for some scheduler hot paths (ttwu).

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
---
kernel/sched.c | 103 ++++++++++++++++++++++++++-------------------------------
1 file changed, 47 insertions(+), 56 deletions(-)
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -600,7 +600,7 @@ static inline int cpu_of(struct rq *rq)
* Return the group to which this tasks belongs.
*
* We use task_subsys_state_check() and extend the RCU verification
- * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
+ * with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach()
* holds that lock for each task it moves into the cgroup. Therefore
* by holding that lock, we pin the task to the current cgroup.
*/
@@ -610,7 +610,7 @@ static inline struct task_group *task_gr
struct cgroup_subsys_state *css;

css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
- lockdep_is_held(&task_rq(p)->lock));
+ lockdep_is_held(&p->pi_lock));
tg = container_of(css, struct task_group, css);

return autogroup_task_group(p, tg);
@@ -926,23 +926,15 @@ static inline void finish_lock_switch(st
#endif /* __ARCH_WANT_UNLOCKED_CTXSW */

/*
- * Check whether the task is waking, we use this to synchronize ->cpus_allowed
- * against ttwu().
- */
-static inline int task_is_waking(struct task_struct *p)
-{
- return unlikely(p->state == TASK_WAKING);
-}
-
-/*
- * __task_rq_lock - lock the runqueue a given task resides on.
- * Must be called interrupts disabled.
+ * __task_rq_lock - lock the rq @p resides on.
*/
static inline struct rq *__task_rq_lock(struct task_struct *p)
__acquires(rq->lock)
{
struct rq *rq;

+ lockdep_assert_held(&p->pi_lock);
+
for (;;) {
rq = task_rq(p);
raw_spin_lock(&rq->lock);
@@ -953,22 +945,22 @@ static inline struct rq *__task_rq_lock(
}

/*
- * task_rq_lock - lock the runqueue a given task resides on and disable
- * interrupts. Note the ordering: we can safely lookup the task_rq without
- * explicitly disabling preemption.
+ * task_rq_lock - lock p->pi_lock and lock the rq @p resides on.
*/
static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
+ __acquires(p->pi_lock)
__acquires(rq->lock)
{
struct rq *rq;

for (;;) {
- local_irq_save(*flags);
+ raw_spin_lock_irqsave(&p->pi_lock, *flags);
rq = task_rq(p);
raw_spin_lock(&rq->lock);
if (likely(rq == task_rq(p)))
return rq;
- raw_spin_unlock_irqrestore(&rq->lock, *flags);
+ raw_spin_unlock(&rq->lock);
+ raw_spin_unlock_irqrestore(&p->pi_lock, *flags);
}
}

@@ -978,10 +970,13 @@ static void __task_rq_unlock(struct rq *
raw_spin_unlock(&rq->lock);
}

-static inline void task_rq_unlock(struct rq *rq, unsigned long *flags)
+static inline void
+task_rq_unlock(struct rq *rq, struct task_struct *p, unsigned long *flags)
__releases(rq->lock)
+ __releases(p->pi_lock)
{
- raw_spin_unlock_irqrestore(&rq->lock, *flags);
+ raw_spin_unlock(&rq->lock);
+ raw_spin_unlock_irqrestore(&p->pi_lock, *flags);
}

/*
@@ -2178,6 +2173,11 @@ void set_task_cpu(struct task_struct *p,
*/
WARN_ON_ONCE(p->state != TASK_RUNNING && p->state != TASK_WAKING &&
!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
+
+#ifdef CONFIG_LOCKDEP
+ WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
+ lockdep_is_held(&task_rq(p)->lock)));
+#endif
#endif

trace_sched_migrate_task(p, new_cpu);
@@ -2273,7 +2273,7 @@ unsigned long wait_task_inactive(struct
ncsw = 0;
if (!match_state || p->state == match_state)
ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);

/*
* If it changed from the expected state, bail out now.
@@ -2639,6 +2639,7 @@ static void __sched_fork(struct task_str
*/
void sched_fork(struct task_struct *p, int clone_flags)
{
+ unsigned long flags;
int cpu = get_cpu();

__sched_fork(p);
@@ -2689,9 +2690,9 @@ void sched_fork(struct task_struct *p, i
*
* Silence PROVE_RCU.
*/
- rcu_read_lock();
+ raw_spin_lock_irqsave(&p->pi_lock, flags);
set_task_cpu(p, cpu);
- rcu_read_unlock();
+ raw_spin_unlock_irqrestore(&p->pi_lock, flags);

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
if (likely(sched_info_on()))
@@ -2740,7 +2741,7 @@ void wake_up_new_task(struct task_struct
set_task_cpu(p, cpu);

p->state = TASK_RUNNING;
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);
#endif

rq = task_rq_lock(p, &flags);
@@ -2751,7 +2752,7 @@ void wake_up_new_task(struct task_struct
if (p->sched_class->task_woken)
p->sched_class->task_woken(rq, p);
#endif
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);
put_cpu();
}

@@ -3476,12 +3477,12 @@ void sched_exec(void)
likely(cpu_active(dest_cpu)) && need_migrate_task(p)) {
struct migration_arg arg = { p, dest_cpu };

- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);
stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
return;
}
unlock:
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);
}

#endif
@@ -3518,7 +3519,7 @@ unsigned long long task_delta_exec(struc

rq = task_rq_lock(p, &flags);
ns = do_task_delta_exec(p, rq);
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);

return ns;
}
@@ -3536,7 +3537,7 @@ unsigned long long task_sched_runtime(st

rq = task_rq_lock(p, &flags);
ns = p->se.sum_exec_runtime + do_task_delta_exec(p, rq);
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);

return ns;
}
@@ -3560,7 +3561,7 @@ unsigned long long thread_group_sched_ru
rq = task_rq_lock(p, &flags);
thread_group_cputime(p, &totals);
ns = totals.sum_exec_runtime + do_task_delta_exec(p, rq);
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);

return ns;
}
@@ -4675,16 +4676,13 @@ EXPORT_SYMBOL(sleep_on_timeout);
*/
void rt_mutex_setprio(struct task_struct *p, int prio)
{
- unsigned long flags;
int oldprio, on_rq, running;
struct rq *rq;
const struct sched_class *prev_class;

BUG_ON(prio < 0 || prio > MAX_PRIO);

- lockdep_assert_held(&p->pi_lock);
-
- rq = task_rq_lock(p, &flags);
+ rq = __task_rq_lock(p);

trace_sched_pi_setprio(p, prio);
oldprio = p->prio;
@@ -4709,7 +4707,7 @@ void rt_mutex_setprio(struct task_struct
enqueue_task(rq, p, oldprio < prio ? ENQUEUE_HEAD : 0);

check_class_changed(rq, p, prev_class, oldprio);
- task_rq_unlock(rq, &flags);
+ __task_rq_unlock(rq);
}

#endif
@@ -4757,7 +4755,7 @@ void set_user_nice(struct task_struct *p
resched_task(rq->curr);
}
out_unlock:
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);
}
EXPORT_SYMBOL(set_user_nice);

@@ -4979,20 +4977,17 @@ static int __sched_setscheduler(struct t
/*
* make sure no PI-waiters arrive (or leave) while we are
* changing the priority of the task:
- */
- raw_spin_lock_irqsave(&p->pi_lock, flags);
- /*
+ *
* To be able to change p->policy safely, the apropriate
* runqueue lock must be held.
*/
- rq = __task_rq_lock(p);
+ rq = task_rq_lock(p, &flags);

/*
* Changing the policy of the stop threads its a very bad idea
*/
if (p == rq->stop) {
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);
return -EINVAL;
}

@@ -5005,8 +5000,7 @@ static int __sched_setscheduler(struct t
if (rt_bandwidth_enabled() && rt_policy(policy) &&
task_group(p)->rt_bandwidth.rt_runtime == 0 &&
!task_group_is_autogroup(task_group(p))) {
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);
return -EPERM;
}
}
@@ -5015,8 +5009,7 @@ static int __sched_setscheduler(struct t
/* recheck policy now with rq lock held */
if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
policy = oldpolicy = -1;
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);
goto recheck;
}
on_rq = p->on_rq;
@@ -5038,8 +5031,7 @@ static int __sched_setscheduler(struct t
activate_task(rq, p, 0);

check_class_changed(rq, p, prev_class, oldprio);
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);

rt_mutex_adjust_pi(p);

@@ -5620,7 +5612,7 @@ SYSCALL_DEFINE2(sched_rr_get_interval, p

rq = task_rq_lock(p, &flags);
time_slice = p->sched_class->get_rr_interval(rq, p);
- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, p, &flags);

rcu_read_unlock();
jiffies_to_timespec(time_slice, &t);
@@ -5843,8 +5835,7 @@ int set_cpus_allowed_ptr(struct task_str
unsigned int dest_cpu;
int ret = 0;

- raw_spin_lock_irqsave(&p->pi_lock, flags);
- rq = __task_rq_lock(p);
+ rq = task_rq_lock(p, &flags);

if (!cpumask_intersects(new_mask, cpu_active_mask)) {
ret = -EINVAL;
@@ -5872,15 +5863,13 @@ int set_cpus_allowed_ptr(struct task_str
if (need_migrate_task(p)) {
struct migration_arg arg = { p, dest_cpu };
/* Need help from migration thread: drop lock and wait. */
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);
stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
tlb_migrate_finish(p->mm);
return 0;
}
out:
- __task_rq_unlock(rq);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ task_rq_unlock(rq, p, &flags);

return ret;
}
@@ -5908,6 +5897,7 @@ static int __migrate_task(struct task_st
rq_src = cpu_rq(src_cpu);
rq_dest = cpu_rq(dest_cpu);

+ raw_spin_lock(&p->pi_lock);
double_rq_lock(rq_src, rq_dest);
/* Already moved. */
if (task_cpu(p) != src_cpu)
@@ -5930,6 +5920,7 @@ static int __migrate_task(struct task_st
ret = 1;
fail:
double_rq_unlock(rq_src, rq_dest);
+ raw_spin_unlock(&p->pi_lock);
return ret;
}

@@ -8656,7 +8647,7 @@ void sched_move_task(struct task_struct
if (on_rq)
enqueue_task(rq, tsk, 0);

- task_rq_unlock(rq, &flags);
+ task_rq_unlock(rq, tsk, &flags);
}
#endif /* CONFIG_CGROUP_SCHED */




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