linux-kernel - Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

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Date:   Wed, 11 Jul 2018 11:20:53 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     peterz@...radead.org, mhillenb@...zon.de,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering
 guest mode

On Wed, Jul 11, 2018 at 07:01:01PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> RCU can spend long periods of time waiting for a CPU which is actually in
> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
> modes, and don't wait for it.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>

And idiot here forgot about some of the debugging code in RCU's dyntick-idle
code.  I will reply with a fixed patch.

The code below works just fine as long as you don't enable CONFIG_RCU_EQS_DEBUG,
so should be OK for testing, just not for mainline.

								Thanx, Paul

> ---
>  arch/x86/kvm/x86.c       | 2 ++
>  include/linux/rcupdate.h | 7 +++++++
>  kernel/rcu/tree.c        | 2 ++
>  3 files changed, 11 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0046aa70205a..b0c82f70afa7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>  	}
> 
> +	rcu_kvm_enter();
>  	kvm_x86_ops->run(vcpu);
> +	rcu_kvm_exit();
> 
>  	/*
>  	 * Do this here before restoring debug registers on the host.  And
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 65163aa0bb04..1325f9d9ce00 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -212,6 +212,13 @@ do { \
>  #error "Unknown RCU implementation specified to kernel configuration"
>  #endif
> 
> +/*
> + * These are currently identical to the _idle_ versions but let's
> + * explicitly have separate copies to keep Paul honest in future.
> + */
> +static inline void rcu_kvm_enter(void) { rcu_idle_enter(); }
> +static inline void rcu_kvm_exit(void) { rcu_idle_exit(); }
> +
>  /*
>   * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
>   * are needed for dynamic initialization and destruction of rcu_head
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index aa7cade1b9f3..d38c381bf4e3 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -760,6 +760,7 @@ void rcu_idle_enter(void)
>  	lockdep_assert_irqs_disabled();
>  	rcu_eqs_enter(false);
>  }
> +EXPORT_SYMBOL_GPL(rcu_idle_enter);
> 
>  #ifdef CONFIG_NO_HZ_FULL
>  /**
> @@ -913,6 +914,7 @@ void rcu_idle_exit(void)
>  	rcu_eqs_exit(false);
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_idle_exit);
> 
>  #ifdef CONFIG_NO_HZ_FULL
>  /**
> -- 
> 2.17.1
>