On Thu, Sep 27, 2012 at 11:33:56AM +0200, Avi Kivity wrote: > On 09/27/2012 11:11 AM, Gleb Natapov wrote: > >> > >> User return notifier is per-cpu, not per-task. There is a new task_work > >> (<linux/task_work.h>) that does what you want. With these > >> technicalities out of the way, I think it's the wrong idea. If a vcpu > >> thread is in userspace, that doesn't mean it's preempted, there's no > >> point in boosting it if it's already running. > >> > > Ah, so you want to set bit in kvm->preempted_vcpus if task is _not_ > > TASK_RUNNING in sched_out (you wrote opposite in your email)? If a task > > is in userspace it is definitely not preempted. > > No, as I originally wrote. If it's TASK_RUNNING when it saw sched_out, > then it is preempted (i.e. runnable), not sleeping on some waitqueue, > voluntarily (HLT) or involuntarily (page fault). > Of course, I got it all backwards. Need more coffee. > > > >> btw, we can have secondary effects. A vcpu can be waiting for a lock in > >> the host kernel, or for a host page fault. There's no point in boosting > >> anything for that. Or a vcpu in userspace can be waiting for a lock > >> that is held by another thread, which has been preempted. > > Do you mean userspace spinlock? Because otherwise task that's waits on > > a kernel lock will sleep in the kernel. > > I meant a kernel mutex. > > vcpu 0: take guest spinlock > vcpu 0: vmexit > vcpu 0: spin_lock(some_lock) > vcpu 1: take same guest spinlock > vcpu 1: PLE vmexit > vcpu 1: wtf? > > Waiting on a host kernel spinlock is not too bad because we expect to be > out shortly. Waiting on a host kernel mutex can be a lot worse. > We can't do much about it without PV spinlock since there is not information about what vcpu holds which guest spinlock, no? -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
