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sched: sched_exec(): Remove the select_fallback_rq() logic 

sched_exec()->select_task_rq() reads/updates ->cpus_allowed lockless.
This can race with other CPUs updating our ->cpus_allowed, and this
looks meaningless to me.

The task is current and running, it must have online cpus in ->cpus_allowed,
the fallback mode is bogus. And, if ->sched_class returns the "wrong" cpu,
this likely means we raced with set_cpus_allowed() which was called
for reason, why should sched_exec() retry and call ->select_task_rq()
again?

Change the code to call sched_class->select_task_rq() directly and do
nothing if the returned cpu is wrong after re-checking under rq->lock.

From now task_struct->cpus_allowed is always stable under TASK_WAKING,
select_fallback_rq() is always called under rq-lock or the caller or
the caller owns TASK_WAKING (select_task_rq).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100315091019.GA9141@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2010-03-15 10:10:19 +01:00 committed by Ingo Molnar
parent c1804d547d
commit 30da688ef6
1 changed files with 8 additions and 17 deletions
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25
kernel/sched.c
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@ -2280,6 +2280,9 @@ void task_oncpu_function_call(struct task_struct *p,
}
#ifdef CONFIG_SMP
/*
* ->cpus_allowed is protected by either TASK_WAKING or rq->lock held.
*/
static int select_fallback_rq(int cpu, struct task_struct *p)
{
int dest_cpu;
@ -2316,12 +2319,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
}
/*
* Gets called from 3 sites (exec, fork, wakeup), since it is called without
* holding rq->lock we need to ensure ->cpus_allowed is stable, this is done
* by:
*
* exec: is unstable, retry loop
* fork & wake-up: serialize ->cpus_allowed against TASK_WAKING
* The caller (fork, wakeup) owns TASK_WAKING, ->cpus_allowed is stable.
*/
static inline
int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
@ -3076,9 +3074,8 @@ void sched_exec(void)
unsigned long flags;
struct rq *rq;
again:
this_cpu = get_cpu();
dest_cpu = select_task_rq(p, SD_BALANCE_EXEC, 0);
dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
if (dest_cpu == this_cpu) {
put_cpu();
return;
@ -3086,18 +3083,12 @@ again:
rq = task_rq_lock(p, &flags);
put_cpu();
/*
* select_task_rq() can race against ->cpus_allowed
*/
if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)
|| unlikely(!cpu_active(dest_cpu))) {
task_rq_unlock(rq, &flags);
goto again;
}
/* force the process onto the specified CPU */
if (migrate_task(p, dest_cpu, &req)) {
if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
likely(cpu_active(dest_cpu)) &&
migrate_task(p, dest_cpu, &req)) {
/* Need to wait for migration thread (might exit: take ref). */
struct task_struct *mt = rq->migration_thread;