linux-kernel - Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

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Date:	Tue, 10 Feb 2015 15:13:22 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>, paulmck@...ux.vnet.ibm.com
CC:	will.deacon@....com, linux-kernel@...r.kernel.org,
	Catalin.Marinas@....com, fweisbec@...il.com, kvm@...r.kernel.org,
	mtosatti@...hat.com, borntraeger@...ibm.com, mingo@...nel.org,
	oleg@...hat.com, lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when
 running KVM guest

On 02/10/2015 02:59 PM, Andy Lutomirski wrote:
> On 02/10/2015 06:41 AM, 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. 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       | 6 ++++++
>>   include/linux/context_tracking_state.h | 1 +
>>   include/linux/kvm_host.h               | 3 ++-
>>   3 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/context_tracking.h
>> b/include/linux/context_tracking.h
>> index 954253283709..b65fd1420e53 100644
>> --- a/include/linux/context_tracking.h
>> +++ b/include/linux/context_tracking.h
>> @@ -80,10 +80,16 @@ static inline void guest_enter(void)
>>           vtime_guest_enter(current);
>>       else
>>           current->flags |= PF_VCPU;
>> +
>> +    if (context_tracking_is_enabled())
>> +        context_tracking_enter(IN_GUEST);
> 
> Why the if statement?
> 
> Also, have you checked how much this hurts guest lightweight entry/exit
> latency?  Context tracking is shockingly expensive for reasons I don't
> fully understand, but hopefully most of it is the vtime stuff.

Guest_enter and guest_exit already do the vtime stuff today.

This patch series adds the rcu stuff, and modifies
context_tracking_enter & context_tracking_exit to not
do the vtime stuff twice.

> (Context tracking is *so* expensive that I almost think we should set the
> performance taint flag if we enable it, assuming that flag ended up
> getting merged.  Also, we should make context tracking faster.)

I am all for making it faster :)

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