sched: Make select_fallback_rq() cpuset friendly
Introduce cpuset_cpus_allowed_fallback() helper to fix the cpuset problems with select_fallback_rq(). It can be called from any context and can't use any cpuset locks including task_lock(). It is called when the task doesn't have online cpus in ->cpus_allowed but ttwu/etc must be able to find a suitable cpu. I am not proud of this patch. Everything which needs such a fat comment can't be good even if correct. But I'd prefer to not change the locking rules in the code I hardly understand, and in any case I believe this simple change make the code much more correct compared to deadlocks we currently have. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <20100315091027.GA9155@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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@ -21,6 +21,7 @@ extern int number_of_cpusets; /* How many cpusets are defined in system? */
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extern int cpuset_init(void);
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extern void cpuset_init_smp(void);
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extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
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extern int cpuset_cpus_allowed_fallback(struct task_struct *p);
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extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
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#define cpuset_current_mems_allowed (current->mems_allowed)
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void cpuset_init_current_mems_allowed(void);
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@ -101,6 +102,12 @@ static inline void cpuset_cpus_allowed(struct task_struct *p,
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cpumask_copy(mask, cpu_possible_mask);
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}
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static inline int cpuset_cpus_allowed_fallback(struct task_struct *p)
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{
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cpumask_copy(&p->cpus_allowed, cpu_possible_mask);
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return cpumask_any(cpu_active_mask);
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}
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static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
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{
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return node_possible_map;
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@ -2188,6 +2188,48 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
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mutex_unlock(&callback_mutex);
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}
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int cpuset_cpus_allowed_fallback(struct task_struct *tsk)
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{
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const struct cpuset *cs;
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int cpu;
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rcu_read_lock();
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cs = task_cs(tsk);
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if (cs)
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cpumask_copy(&tsk->cpus_allowed, cs->cpus_allowed);
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rcu_read_unlock();
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/*
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* We own tsk->cpus_allowed, nobody can change it under us.
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*
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* But we used cs && cs->cpus_allowed lockless and thus can
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* race with cgroup_attach_task() or update_cpumask() and get
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* the wrong tsk->cpus_allowed. However, both cases imply the
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* subsequent cpuset_change_cpumask()->set_cpus_allowed_ptr()
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* which takes task_rq_lock().
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*
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* If we are called after it dropped the lock we must see all
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* changes in tsk_cs()->cpus_allowed. Otherwise we can temporary
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* set any mask even if it is not right from task_cs() pov,
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* the pending set_cpus_allowed_ptr() will fix things.
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*/
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cpu = cpumask_any_and(&tsk->cpus_allowed, cpu_active_mask);
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if (cpu >= nr_cpu_ids) {
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/*
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* Either tsk->cpus_allowed is wrong (see above) or it
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* is actually empty. The latter case is only possible
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* if we are racing with remove_tasks_in_empty_cpuset().
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* Like above we can temporary set any mask and rely on
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* set_cpus_allowed_ptr() as synchronization point.
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*/
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cpumask_copy(&tsk->cpus_allowed, cpu_possible_mask);
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cpu = cpumask_any(cpu_active_mask);
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}
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return cpu;
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}
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void cpuset_init_current_mems_allowed(void)
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{
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nodes_setall(current->mems_allowed);
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@ -2300,9 +2300,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
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/* No more Mr. Nice Guy. */
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if (unlikely(dest_cpu >= nr_cpu_ids)) {
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cpumask_copy(&p->cpus_allowed, cpu_possible_mask);
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dest_cpu = cpumask_any(cpu_active_mask);
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dest_cpu = cpuset_cpus_allowed_fallback(p);
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/*
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* Don't tell them about moving exiting tasks or
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* kernel threads (both mm NULL), since they never
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