linux-kernel - Re: [PATCH 4/5] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

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Message-ID: <20150206232439.GB18934@lerouge>
Date:	Sat, 7 Feb 2015 00:24:41 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	riel@...hat.com
Cc:	kvm@...r.kernel.org, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.orgm, ak@...ux.intel.com, oleg@...hat.com,
	masami.hiramatsu.pt@...achi.com, paulmck@...ux.vnet.ibm.com,
	lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 4/5] kvm,rcu,nohz: use RCU extended quiescent state when
 running KVM guest

On Thu, Feb 05, 2015 at 03:23:51PM -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, so leave those calls
> where they are, instead of moving them into the context tracking
> code.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  include/linux/context_tracking.h       | 8 +++++++-
>  include/linux/context_tracking_state.h | 1 +
>  include/linux/kvm_host.h               | 3 ++-
>  3 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index bd9f000fc98d..a5d3bb44b897 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -43,7 +43,7 @@ static inline enum ctx_state exception_enter(void)
>  static inline void exception_exit(enum ctx_state prev_ctx)
>  {
>  	if (context_tracking_is_enabled()) {
> -		if (prev_ctx == IN_USER)
> +		if (prev_ctx == IN_USER || prev_ctx == IN_GUEST)

That's nitpicking but != IN_KERNEL would be more generic. We are exiting an exception
and we know that the exception executes IN_KERNEL, so we want to restore any context
(whether IN_USER, IN_GUEST, or anything added in the future) prior the exception if that
was anything else than IN_KERNEL.

>  			context_tracking_user_enter(prev_ctx);
>  	}
>  }
> @@ -78,6 +78,9 @@ static inline void guest_enter(void)
>  		vtime_guest_enter(current);
>  	else
>  		current->flags |= PF_VCPU;
> +
> +	if (context_tracking_is_enabled())
> +		context_tracking_user_enter(IN_GUEST);
>  }
>  
>  static inline void guest_exit(void)
> @@ -86,6 +89,9 @@ static inline void guest_exit(void)
>  		vtime_guest_exit(current);
>  	else
>  		current->flags &= ~PF_VCPU;
> +
> +	if (context_tracking_is_enabled())
> +		context_tracking_user_exit(IN_GUEST);

I suggest you to restore RCU before anything else. I believe cputime
accounting doesn't use RCU but we never know with all the debug/tracing
code behind, the acct accounting...

Thanks.

>  }
>  
>  #else
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 97a81225d037..f3ef027af749 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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