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Subject[RFC][PATCH 08/17] sched: Drop the rq argument to sched_class::select_task_rq()
In preparation of calling select_task_rq() without rq->lock held, drop
the dependency on the rq argument.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/sched.h | 3 +--
kernel/sched.c | 40 ++++++++++++++--------------------------
kernel/sched_fair.c | 2 +-
kernel/sched_idletask.c | 2 +-
kernel/sched_rt.c | 10 +++++++++-
kernel/sched_stoptask.c | 3 +--
6 files changed, 27 insertions(+), 33 deletions(-)

Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1063,8 +1063,7 @@ struct sched_class {
void (*put_prev_task) (struct rq *rq, struct task_struct *p);

#ifdef CONFIG_SMP
- int (*select_task_rq)(struct rq *rq, struct task_struct *p,
- int sd_flag, int flags);
+ int (*select_task_rq)(struct task_struct *p, int sd_flag, int flags);

void (*pre_schedule) (struct rq *this_rq, struct task_struct *task);
void (*post_schedule) (struct rq *this_rq);
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2138,13 +2138,14 @@ static int migration_cpu_stop(void *data
* The task's runqueue lock must be held.
* Returns true if you have to wait for migration thread.
*/
-static bool migrate_task(struct task_struct *p, struct rq *rq)
+static bool need_migrate_task(struct task_struct *p)
{
/*
* If the task is not on a runqueue (and not running), then
* the next wake-up will properly place the task.
*/
- return p->on_rq || task_running(rq, p);
+ smp_rmb(); /* finish_lock_switch() */
+ return p->on_rq || p->on_cpu;
}

/*
@@ -2337,9 +2338,9 @@ static int select_fallback_rq(int cpu, s
* The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
*/
static inline
-int select_task_rq(struct rq *rq, struct task_struct *p, int sd_flags, int wake_flags)
+int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
{
- int cpu = p->sched_class->select_task_rq(rq, p, sd_flags, wake_flags);
+ int cpu = p->sched_class->select_task_rq(p, sd_flags, wake_flags);

/*
* In order not to call set_task_cpu() on a blocking task we need
@@ -2484,7 +2485,7 @@ static int try_to_wake_up(struct task_st
en_flags |= ENQUEUE_WAKING;
}

- cpu = select_task_rq(rq, p, SD_BALANCE_WAKE, wake_flags);
+ cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
if (cpu != orig_cpu)
set_task_cpu(p, cpu);
__task_rq_unlock(rq);
@@ -2680,24 +2681,17 @@ void wake_up_new_task(struct task_struct
{
unsigned long flags;
struct rq *rq;
- int cpu __maybe_unused = get_cpu();

#ifdef CONFIG_SMP
rq = task_rq_lock(p, &flags);
- p->state = TASK_WAKING;

/*
* Fork balancing, do it here and not earlier because:
* - cpus_allowed can change in the fork path
* - any previously selected cpu might disappear through hotplug
- *
- * We set TASK_WAKING so that select_task_rq() can drop rq->lock
- * without people poking at ->cpus_allowed.
*/
- cpu = select_task_rq(rq, p, SD_BALANCE_FORK, 0);
- set_task_cpu(p, cpu);
+ set_task_cpu(p, select_task_rq(p, SD_BALANCE_FORK, 0));

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

@@ -2710,7 +2704,6 @@ void wake_up_new_task(struct task_struct
p->sched_class->task_woken(rq, p);
#endif
task_rq_unlock(rq, &flags);
- put_cpu();
}

#ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -3416,27 +3409,22 @@ void sched_exec(void)
{
struct task_struct *p = current;
unsigned long flags;
- struct rq *rq;
int dest_cpu;

- rq = task_rq_lock(p, &flags);
- dest_cpu = p->sched_class->select_task_rq(rq, p, SD_BALANCE_EXEC, 0);
+ raw_spin_lock_irqsave(&p->pi_lock, flags);
+ dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
if (dest_cpu == smp_processor_id())
goto unlock;

- /*
- * select_task_rq() can race against ->cpus_allowed
- */
- if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
- likely(cpu_active(dest_cpu)) && migrate_task(p, rq)) {
+ if (likely(cpu_active(dest_cpu)) && need_migrate_task(p)) {
struct migration_arg arg = { p, dest_cpu };

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

#endif
@@ -5681,7 +5669,7 @@ int set_cpus_allowed_ptr(struct task_str
goto out;

dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
- if (migrate_task(p, rq)) {
+ 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);
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -1623,7 +1623,7 @@ static int select_idle_sibling(struct ta
* preempt must be disabled.
*/
static int
-select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_flags)
+select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
{
struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
int cpu = smp_processor_id();
Index: linux-2.6/kernel/sched_idletask.c
===================================================================
--- linux-2.6.orig/kernel/sched_idletask.c
+++ linux-2.6/kernel/sched_idletask.c
@@ -7,7 +7,7 @@

#ifdef CONFIG_SMP
static int
-select_task_rq_idle(struct rq *rq, struct task_struct *p, int sd_flag, int flags)
+select_task_rq_idle(struct task_struct *p, int sd_flag, int flags)
{
return task_cpu(p); /* IDLE tasks as never migrated */
}
Index: linux-2.6/kernel/sched_rt.c
===================================================================
--- linux-2.6.orig/kernel/sched_rt.c
+++ linux-2.6/kernel/sched_rt.c
@@ -973,11 +973,18 @@ static void yield_task_rt(struct rq *rq)
static int find_lowest_rq(struct task_struct *task);

static int
-select_task_rq_rt(struct rq *rq, struct task_struct *p, int sd_flag, int flags)
+select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
{
if (sd_flag != SD_BALANCE_WAKE)
return smp_processor_id();

+#if 0
+ /*
+ * XXX without holding rq->lock the below is racy, need to
+ * rewrite it in a racy but non-dangerous way so that we mostly
+ * get the benefit of the heuristic but don't crash the kernel
+ * if we get it wrong ;-)
+ */
/*
* If the current task is an RT task, then
* try to see if we can wake this RT task up on another
@@ -1002,6 +1009,7 @@ select_task_rq_rt(struct rq *rq, struct

return (cpu == -1) ? task_cpu(p) : cpu;
}
+#endif

/*
* Otherwise, just let it ride on the affined RQ and the
Index: linux-2.6/kernel/sched_stoptask.c
===================================================================
--- linux-2.6.orig/kernel/sched_stoptask.c
+++ linux-2.6/kernel/sched_stoptask.c
@@ -9,8 +9,7 @@

#ifdef CONFIG_SMP
static int
-select_task_rq_stop(struct rq *rq, struct task_struct *p,
- int sd_flag, int flags)
+select_task_rq_stop(struct task_struct *p, int sd_flag, int flags)
{
return task_cpu(p); /* stop tasks as never migrate */
}



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