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Commit 30da688e authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar
Browse files

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: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100315091019.GA9141@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent c1804d54

kernel/sched.c

+8 −17
Original line number Diff line number Diff line
@@ -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 @@ void sched_exec(void) 


	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;