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

[Christian Borntraeger]

On 07/12/2018 01:37 AM, Paul E. McKenney wrote:
> On Wed, Jul 11, 2018 at 02:32:59PM -0700, Paul E. McKenney wrote:
>> On Wed, Jul 11, 2018 at 11:11:19PM +0200, Christian Borntraeger wrote:
>>>
>>>
>>> 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@xxxxxxxxxxxx>
>>>>>>>>
>>>>>>>> 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@xxxxxxxxxxxx>
>>>>>>>
>>>>>>> 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@xxxxxxxxxxxx>
>>>>>> 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@xxxxxxxxxxxx>
>>>>>>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>>>>>>     [ 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.
>>
>> And my second patch on the email you replied to replaced the only call
>> to rcu_virt_note_context_switch().  So maybe it covers what it needs to,
>> but yes, there might well be things I missed.  Let's see what David
>> comes up with.
>>
>> What changed was RCU's reactions to longish grace periods.  It used to
>> be very aggressive about forcing the scheduler to do otherwise-unneeded
>> context switches, which became a problem somewhere between v4.9 and v4.15.
>> I therefore reduced the number of such context switches, which in turn
>> caused KVM to tell RCU about quiescent states way too infrequently.
>>
>> The advantage of the rcu_kvm_enter()/rcu_kvm_exit() approach is that
>> it tells RCU of an extended duration in the guest, which means that
>> RCU can ignore the corresponding CPU, which in turn allows the guest
>> to proceed without any RCU-induced interruptions.
>>
>> Does that make sense, or am I missing something?  I freely admit to
>> much ignorance of both kvm and s390!  ;-)
> 
> But I am getting some rcutorture near misses on the commit that
> introduces rcu_kvm_enter() and rcu_kvm_exit() to the x86 arch-specific
> vcpu_enter_guest() function.  These near misses occur when running
> rcutorture scenarios TREE01 and TREE03, and in my -rcu tree rather
> than the v4.15 version of this patch.
> 
> Given that I am making pervasive changes to the way that RCU works,
> it might well be that this commit is an innocent bystander.  I will
> run tests overnight and let you know what comes up.

Is there a single patch that that I can test or do I have to combine all the pieces
that are sprinkled in this mail thread?