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

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Message-Id: <2f7ba67e-0442-13cc-628c-2dca56520c21@de.ibm.com>
Date:   Wed, 11 Jul 2018 23:11:19 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     David Woodhouse <dwmw2@...radead.org>, 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 10:27 PM, Paul E. McKenney wrote:
> On Wed, Jul 11, 2018 at 08:39:36PM +0200, Christian Borntraeger wrote:
>>
>>
>> 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.
> 
> Something like this, on top of the earlier patch?  I am not at all
> confident of this patch because there might be other entry/exit
> paths I am missing.  Plus there might be RCU uses on the arch-specific
> patch to and from the guest OS.
> 
> Thoughts?
> 

If you instrment guest_enter/exit, you should cover all cases and all architectures as far
as I can tell. FWIW, we did this rcu_note thing back then actually handling this particular
case of long running guests blocking rcu for many seconds. And I am pretty sure that
this did help back then.