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

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Message-Id: <62e945ee-a58e-f9b3-279c-74cd0f5809da@de.ibm.com>
Date:   Wed, 11 Jul 2018 20:39:36 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     paulmck@...ux.vnet.ibm.com, 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 07/11/2018 08:36 PM, Paul E. McKenney wrote:
> On Wed, Jul 11, 2018 at 11:20:53AM -0700, Paul E. McKenney wrote:
>> 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.
> 
> And here is the updated code that allegedly avoids splatting when run with
> CONFIG_RCU_EQS_DEBUG.
> 
> Thoughts?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 12cd59e49cf734f907f44b696e2c6e4b46a291c3
> Author: David Woodhouse <dwmw@...zon.co.uk>
> Date:   Wed Jul 11 19:01:01 2018 +0100
> 
>     kvm/x86: Inform RCU of quiescent state when entering guest mode
>     
>     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>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>     [ paulmck: Adjust to avoid bad advice I gave to dwmw, avoid WARN_ON()s. ]
> 
> 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();

As indicated in my other mail. This is supposed to be handled in the guest_enter|exit_ calls around
the run function. This would also handle other architectures. So if the guest_enter_irqoff code is
not good enough, we should rather fix that instead of adding another rcu hint.