2010-12-16 15:15:22

by Peter Zijlstra

Subject: [RFC][PATCH 1/5] sched: Always provide p->oncpu

Always provide p->oncpu so that we can determine if its on a cpu
without having to lock the rq.

Signed-off-by: Peter Zijlstra <[email protected]>
---
include/linux/sched.h | 2 --
kernel/sched.c | 36 ++++++++++++++++++++++++------------
2 files changed, 24 insertions(+), 14 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -850,18 +850,39 @@ static inline int task_current(struct rq
return rq->curr == p;
}

-#ifndef __ARCH_WANT_UNLOCKED_CTXSW
static inline int task_running(struct rq *rq, struct task_struct *p)
{
+#ifdef CONFIG_SMP
+ return p->oncpu;
+#else
return task_current(rq, p);
+#endif
}

+#ifndef __ARCH_WANT_UNLOCKED_CTXSW
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->oncpu = 1;
+#endif
}

static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
{
+#ifdef CONFIG_SMP
+ /*
+ * After ->oncpu is cleared, the task can be moved to a different CPU.
+ * We must ensure this doesn't happen until the switch is completely
+ * finished.
+ */
+ smp_wmb();
+ prev->oncpu = 0;
+#endif
#ifdef CONFIG_DEBUG_SPINLOCK
/* this is a valid case when another task releases the spinlock */
rq->lock.owner = current;
@@ -877,15 +898,6 @@ static inline void finish_lock_switch(st
}

#else /* __ARCH_WANT_UNLOCKED_CTXSW */
-static inline int task_running(struct rq *rq, struct task_struct *p)
-{
-#ifdef CONFIG_SMP
- return p->oncpu;
-#else
- return task_current(rq, p);
-#endif
-}
-
static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
{
#ifdef CONFIG_SMP
@@ -2589,7 +2601,7 @@ void sched_fork(struct task_struct *p, i
if (likely(sched_info_on()))
memset(&p->sched_info, 0, sizeof(p->sched_info));
#endif
-#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
+#if defined(CONFIG_SMP)
p->oncpu = 0;
#endif
#ifdef CONFIG_PREEMPT
@@ -5502,7 +5514,7 @@ void __cpuinit init_idle(struct task_str
rcu_read_unlock();

rq->curr = rq->idle = idle;
-#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
+#if defined(CONFIG_SMP)
idle->oncpu = 1;
#endif
raw_spin_unlock_irqrestore(&rq->lock, flags);
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1198,10 +1198,8 @@ struct task_struct {
int lock_depth; /* BKL lock depth */

#ifdef CONFIG_SMP
-#ifdef __ARCH_WANT_UNLOCKED_CTXSW
int oncpu;
#endif
-#endif

int prio, static_prio, normal_prio;
unsigned int rt_priority;

2010-12-18 01:03:47

by Frank Rowand

Subject: Re: [RFC][PATCH 1/5] sched: Always provide p->oncpu

On 12/16/10 06:56, Peter Zijlstra wrote:
> Always provide p->oncpu so that we can determine if its on a cpu
> without having to lock the rq.
>
> Signed-off-by: Peter Zijlstra <[email protected]>

Looks good to me.

Acked-by: Frank Rowand <[email protected]>