Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

From: Andy Lutomirski
Date: Tue Feb 10 2015 - 14:59:18 EST

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

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

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)
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.)

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