linux-kernel - Re: [PATCH 5/5] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

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Date:	Tue, 10 Feb 2015 13:42:06 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	riel@...hat.com
Cc:	fweisbec@...il.com, luto@...capital.net, will.deacon@....com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mtosatti@...hat.com, borntraeger@...ibm.com,
	lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 5/5] kvm,rcu,nohz: use RCU extended quiescent state when
 running KVM guest

On Tue, Feb 10, 2015 at 03:27:54PM -0500, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.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@...hat.com>

The checks for context_tracking_is_enabled() around the calls to
context_tracking_enter() and context_tracking_exit() (as opposed to
only within these functions) looked strange at first, but it avoids
a needless unconditional call in cases where the static_key disables
context tracking.  (This is due to separate compilation.)

So...

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.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);
>  }
> 
>  static inline void guest_exit(void)
>  {
> +	if (context_tracking_is_enabled())
> +		context_tracking_exit(IN_GUEST);
> +
>  	if (vtime_accounting_enabled())
>  		vtime_guest_exit(current);
>  	else
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 72ab10fe1e46..90a7bab8779e 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -15,6 +15,7 @@ struct context_tracking {
>  	enum ctx_state {
>  		IN_KERNEL = 0,
>  		IN_USER,
> +		IN_GUEST,
>  	} state;
>  };
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 26f106022c88..c7828a6a9614 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -772,7 +772,8 @@ static inline void kvm_guest_enter(void)
>  	 * one time slice). Lets treat guest mode as quiescent state, just like
>  	 * we do with user-mode execution.
>  	 */
> -	rcu_virt_note_context_switch(smp_processor_id());
> +	if (!context_tracking_cpu_is_enabled())
> +		rcu_virt_note_context_switch(smp_processor_id());
>  }
> 
>  static inline void kvm_guest_exit(void)
> -- 
> 1.9.3
> 

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