On Thu, Jun 29, 2017 at 09:03:42PM -0700, Linus Torvalds wrote: > On Thu, Jun 29, 2017 at 12:15 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Thu, Jun 29, 2017 at 09:13:29AM -0700, Linus Torvalds wrote: > >> > >> swait uses special locking and has odd semantics that are not at all > >> the same as the default wait queue ones. It should not be used without > >> very strong reasons (and honestly, the only strong enough reason seems > >> to be "RT"). > > > > Performance shortcut: > > > > https://lkml.org/lkml/2016/2/25/301 > > Yes, I know why kvm uses it, I just don't think it's necessarily the > right thing. > > That kvm commit is actually a great example: it uses swake_up() from > an interrupt, and that's in fact the *reason* it uses swake_up(). > > But that also fundamentally means that it cannot use swake_up_all(), > so it basically *relies* on there only ever being one single entry > that needs to be woken up. > > And as far as I can tell, it really is because the queue only ever has > one entry (ie it's per-vcpu, and when the vcpu is blocked, it's > blocked - so no other user will be waiting there). Exactly. > > So it isn't that you migth queue multiple entries and then just wake > them up one at a time. There really is just one entry at a time, > right? Yes. > And that means that swait is actuially completely the wrong thing to > do. It's more expensive and more complex than just saving the single > process pointer away and just doing "wake_up_process()". Aha, i see. > > Now, it really is entirely possible that I'm missing something, but it > does look like that to me. Just drop it -- the optimization is not relevant anymore given VMX hardware improvements. > We've had wake_up_process() since pretty much day #1. THAT is the > fastest and simplest direct wake-up there is, not some "simple > wait-queue". > > Now, admittedly I don't know the code and really may be entirely off, > but looking at the commit (no need to go to the lkml archives - it's > commit 8577370fb0cb ("KVM: Use simple waitqueue for vcpu->wq") in > mainline), I really think the swait() use is simply not correct if > there can be multiple waiters, exactly because swake_up() only wakes > up a single entry. There can't be: its one emulated LAPIC per vcpu. So only one vcpu waits for that waitqueue. > So either there is only a single entry, or *all* the code like > > dvcpu->arch.wait = 0; > > - if (waitqueue_active(&dvcpu->wq)) > - wake_up_interruptible(&dvcpu->wq); > + if (swait_active(&dvcpu->wq)) > + swake_up(&dvcpu->wq); > > is simply wrong. If there are multiple blockers, and you just cleared > "arch.wait", I think they should *all* be woken up. And that's not > what swake_up() does. > > So I think that kvm_vcpu_block() could easily have instead done > > vcpu->process = current; > > as the "prepare_to_wait()" part, and "finish_wait()" would be to just > clear vcpu->process. No wait-queue, just a single pointer to the > single blocking thread. > > (Of course, you still need serialization, so that > "wake_up_process(vcpu->process)" doesn't end up using a stale value, > but since processes are already freed with RCU because of other things > like that, the serialization is very low-cost, you only need to be > RCU-read safe when waking up). > > See what I'm saying? > > Note that "wake_up_process()" really is fairly widely used. It's > widely used because it's fairly obvious, and because that really *is* > the lowest-possible cost: a single pointer to the sleeping thread, and > you can often do almost no locking at all. > > And unlike swake_up(), it's obvious that you only wake up a single thread. > > Linus Feel free to drop the KVM usage... agreed the interface is a special case and a generic one which handles multiple waiters and has debugging etc should be preferred to avoid bugs Not sure if other people are using it (swait).
