On Tue, Feb 10, 2015 at 11:59:09AM -0800, Andy Lutomirski wrote: > On 02/10/2015 06:41 AM, riel@xxxxxxxxxx wrote: > >From: Rik van Riel <riel@xxxxxxxxxx> > > > >The host kernel is not doing anything while the CPU is executing > >a KVM guest VCPU, so it can be marked as being in an extended > >quiescent state, identical to that used when running user space > >code. > > > >The only exception to that rule is when the host handles an > >interrupt, which is already handled by the irq code, which > >calls rcu_irq_enter and rcu_irq_exit. > > > >The guest_enter and guest_exit functions already switch vtime > >accounting independent of context tracking. Leave those calls > >where they are, instead of moving them into the context tracking > >code. > > > >Signed-off-by: Rik van Riel <riel@xxxxxxxxxx> > >--- > > include/linux/context_tracking.h | 6 ++++++ > > include/linux/context_tracking_state.h | 1 + > > include/linux/kvm_host.h | 3 ++- > > 3 files changed, 9 insertions(+), 1 deletion(-) > > > >diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h > >index 954253283709..b65fd1420e53 100644 > >--- a/include/linux/context_tracking.h > >+++ b/include/linux/context_tracking.h > >@@ -80,10 +80,16 @@ static inline void guest_enter(void) > > vtime_guest_enter(current); > > else > > current->flags |= PF_VCPU; > >+ > >+ if (context_tracking_is_enabled()) > >+ context_tracking_enter(IN_GUEST); > > Why the if statement? > > Also, have you checked how much this hurts guest lightweight > entry/exit latency? Context tracking is shockingly expensive for > reasons I don't fully understand, but hopefully most of it is the > vtime stuff. (Context tracking is *so* expensive that I almost > think we should set the performance taint flag if we enable it, > assuming that flag ended up getting merged. Also, we should make > context tracking faster.) It turns out that context_tracking_is_enabled() is a static inline that uses a static_key, so the overhead should be minimal on platforms having a full implementation of static keys. Thanx, Paul -- 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
