sched: Fix hotplug task migration
Dan Carpenter reported:
> kernel/sched/rt.c:1347 pick_next_task_rt() warn: variable dereferenced before check 'prev' (see line 1338)
> kernel/sched/deadline.c:1011 pick_next_task_dl() warn: variable dereferenced before check 'prev' (see line 1005)
Kirill also spotted that migrate_tasks() will have an instant NULL
deref because pick_next_task() will immediately deref prev.
Instead of fixing all the corner cases because migrate_tasks() can
pass in a NULL prev task in the unlikely case of hot-un-plug, provide
a fake task such that we can remove all the NULL checks from the far
more common paths.
A further problem; not previously spotted; is that because we pushed
pre_schedule() and idle_balance() into pick_next_task() we now need to
avoid those getting called and pulling more tasks on our dying CPU.
We avoid pull_{dl,rt}_task() by setting fake_task.prio to MAX_PRIO+1.
We also note that since we call pick_next_task() exactly the amount of
times we have runnable tasks present, we should never land in
idle_balance().
Fixes: 38033c37fa
("sched: Push down pre_schedule() and idle_balance()")
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Kirill Tkhai <tkhai@yandex.ru>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140212094930.GB3545@laptop.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
parent
6e83125c6b
commit
3f1d2a3181
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@ -4681,6 +4681,22 @@ static void calc_load_migrate(struct rq *rq)
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atomic_long_add(delta, &calc_load_tasks);
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atomic_long_add(delta, &calc_load_tasks);
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}
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}
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static void put_prev_task_fake(struct rq *rq, struct task_struct *prev)
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{
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}
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static const struct sched_class fake_sched_class = {
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.put_prev_task = put_prev_task_fake,
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};
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static struct task_struct fake_task = {
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/*
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* Avoid pull_{rt,dl}_task()
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*/
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.prio = MAX_PRIO + 1,
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.sched_class = &fake_sched_class,
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};
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/*
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/*
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* Migrate all tasks from the rq, sleeping tasks will be migrated by
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* Migrate all tasks from the rq, sleeping tasks will be migrated by
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* try_to_wake_up()->select_task_rq().
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* try_to_wake_up()->select_task_rq().
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@ -4721,7 +4737,7 @@ static void migrate_tasks(unsigned int dead_cpu)
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if (rq->nr_running == 1)
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if (rq->nr_running == 1)
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break;
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break;
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next = pick_next_task(rq, NULL);
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next = pick_next_task(rq, &fake_task);
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BUG_ON(!next);
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BUG_ON(!next);
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next->sched_class->put_prev_task(rq, next);
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next->sched_class->put_prev_task(rq, next);
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@ -1008,8 +1008,7 @@ struct task_struct *pick_next_task_dl(struct rq *rq, struct task_struct *prev)
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if (unlikely(!dl_rq->dl_nr_running))
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if (unlikely(!dl_rq->dl_nr_running))
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return NULL;
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return NULL;
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if (prev)
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put_prev_task(rq, prev);
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prev->sched_class->put_prev_task(rq, prev);
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dl_se = pick_next_dl_entity(rq, dl_rq);
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dl_se = pick_next_dl_entity(rq, dl_rq);
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BUG_ON(!dl_se);
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BUG_ON(!dl_se);
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@ -4690,7 +4690,7 @@ again:
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if (!cfs_rq->nr_running)
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if (!cfs_rq->nr_running)
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goto idle;
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goto idle;
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if (!prev || prev->sched_class != &fair_sched_class)
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if (prev->sched_class != &fair_sched_class)
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goto simple;
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goto simple;
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/*
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/*
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@ -4766,8 +4766,7 @@ simple:
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if (!cfs_rq->nr_running)
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if (!cfs_rq->nr_running)
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goto idle;
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goto idle;
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if (prev)
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put_prev_task(rq, prev);
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prev->sched_class->put_prev_task(rq, prev);
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do {
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do {
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se = pick_next_entity(cfs_rq, NULL);
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se = pick_next_entity(cfs_rq, NULL);
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@ -26,8 +26,7 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
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static struct task_struct *
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static struct task_struct *
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pick_next_task_idle(struct rq *rq, struct task_struct *prev)
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pick_next_task_idle(struct rq *rq, struct task_struct *prev)
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{
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{
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if (prev)
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put_prev_task(rq, prev);
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prev->sched_class->put_prev_task(rq, prev);
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schedstat_inc(rq, sched_goidle);
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schedstat_inc(rq, sched_goidle);
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#ifdef CONFIG_SMP
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#ifdef CONFIG_SMP
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@ -1344,8 +1344,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
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if (rt_rq_throttled(rt_rq))
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if (rt_rq_throttled(rt_rq))
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return NULL;
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return NULL;
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if (prev)
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put_prev_task(rq, prev);
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prev->sched_class->put_prev_task(rq, prev);
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p = _pick_next_task_rt(rq);
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p = _pick_next_task_rt(rq);
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@ -1147,6 +1147,11 @@ struct sched_class {
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#endif
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#endif
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};
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};
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static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
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{
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prev->sched_class->put_prev_task(rq, prev);
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}
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#define sched_class_highest (&stop_sched_class)
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#define sched_class_highest (&stop_sched_class)
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#define for_each_class(class) \
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#define for_each_class(class) \
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for (class = sched_class_highest; class; class = class->next)
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for (class = sched_class_highest; class; class = class->next)
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@ -31,8 +31,7 @@ pick_next_task_stop(struct rq *rq, struct task_struct *prev)
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if (!stop || !stop->on_rq)
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if (!stop || !stop->on_rq)
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return NULL;
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return NULL;
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if (prev)
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put_prev_task(rq, prev);
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prev->sched_class->put_prev_task(rq, prev);
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stop->se.exec_start = rq_clock_task(rq);
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stop->se.exec_start = rq_clock_task(rq);
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