On Tue, Feb 10, 2015 at 12:14 PM, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > 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. Shouldn't we just fold that into context_tracking_xyz_enter? Also, why does the vtime stuff depend on RCU extended quiescent states? To me, they seem mostly orthogonal other than the fact that they hook into the same places. --Andy -- 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
