内核进行进程切换时，先调用了__schedule，在关抢占后调用context_switch：
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static void __sched __schedule(void) { struct task_struct *prev, *next; unsigned long *switch_count; struct rq *rq; int cpu; need_resched: preempt_disable(); cpu = smp_processor_id(); rq = cpu_rq(cpu); ... raw_spin_lock_irq(&rq->lock); ... context_switch(rq, prev, next); ... sched_preempt_enable_no_resched(); |
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static inline void context_switch(struct rq *rq, struct task_struct *prev, struct task_struct *next) { struct mm_struct *mm, *oldmm; prepare_task_switch(rq, prev, next); mm = next->mm; oldmm = prev->active_mm; … finish_task_switch(this_rq(), prev); |
prepare_task_switch里面调用fire_sched_out_preempt_notifiers，进而调用prev进程注册的sched_out操作，
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static inline void prepare_task_switch(struct rq *rq, struct task_struct *prev, struct task_struct *next) { trace_sched_switch(prev, next); sched_info_switch(prev, next); perf_event_task_sched_out(prev, next); fire_sched_out_preempt_notifiers(prev, next); prepare_lock_switch(rq, next); prepare_arch_switch(next); } |
这里是分支：如果是新创建的进程被调度了，要调用schedule_tail：
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/** * schedule_tail - first thing a freshly forked thread must call. * @prev: the thread we just switched away from. */ asmlinkage void schedule_tail(struct task_struct *prev) __releases(rq->lock) { struct rq *rq = this_rq(); finish_task_switch(rq, prev); /* * FIXME: do we need to worry about rq being invalidated by the * task_switch? */ post_schedule(rq); #ifdef __ARCH_WANT_UNLOCKED_CTXSW /* In this case, finish_task_switch does not reenable preemption */ preempt_enable(); #endif if (current->set_child_tid) put_user(task_pid_vnr(current), current->set_child_tid); } |
回到主题：finish_task_switch这里会调用fire_sched_in_preempt_notifiers:
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static void fire_sched_in_preempt_notifiers(struct task_struct *curr) { struct preempt_notifier *notifier; hlist_for_each_entry(notifier, &curr->preempt_notifiers, link) notifier->ops->sched_in(notifier, raw_smp_processor_id()); } |
preempt_notifiers在哪里注册的呢，对kvm来说是这里：
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static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id) { int r; struct kvm_vcpu *vcpu, *v; if (id >= KVM_MAX_VCPUS) return -EINVAL; vcpu = kvm_arch_vcpu_create(kvm, id); if (IS_ERR(vcpu)) return PTR_ERR(vcpu); preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops); r = kvm_arch_vcpu_setup(vcpu); |
kvm模块加载的时候vmx_init->kvm_init里面初始化了kvm_preempt_ops
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kvm_preempt_ops.sched_in = kvm_sched_in; kvm_preempt_ops.sched_out = kvm_sched_out; |
所以kvm_sched_in和kvm_sched_out被调用时的上下文是关抢占的