| From 1c6ed13f6a3016de8e8ed05bf96c684aac51fea9 Mon Sep 17 00:00:00 2001 |
| From: Oleg Nesterov <oleg@redhat.com> |
| Date: Mon, 15 Mar 2010 10:10:19 +0100 |
| Subject: [PATCH] sched: sched_exec(): Remove the select_fallback_rq() logic |
| |
| commit a325588ec5c7384514a79f01ddebf274477eaf51 in tip. |
| |
| 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). |
| |
| [ upstream commit: 30da688ef6b76e01969b00608202fff1eed2accc ] |
| |
| 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> |
| Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
| |
| diff --git a/kernel/sched.c b/kernel/sched.c |
| index dfc2d66..adad328 100644 |
| --- a/kernel/sched.c |
| +++ b/kernel/sched.c |
| @@ -2379,6 +2379,9 @@ 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; |
| @@ -2415,12 +2418,7 @@ 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) |
| @@ -3338,9 +3336,8 @@ 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; |
| @@ -3348,18 +3345,12 @@ 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; |
| |
| -- |
| 1.7.1.1 |
| |