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

[Andy Lutomirski <luto@amacapital.net>]

On 02/10/2015 06:41 AM, riel@redhat.com wrote:
> From: Rik van Riel <riel@redhat.com>
>
> 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@redhat.com>
> ---
>   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.)

--Andy