linux-kernel - Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest

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Date:	Thu, 05 Feb 2015 11:52:37 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>, kvm@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.org, ak@...ux.intel.com, oleg@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	paulmck@...ux.vnet.ibm.com, lcapitulino@...hat.com,
	pbonzini@...hat.com
Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running
 KVM guest

On 02/05/2015 11:44 AM, Christian Borntraeger wrote:
> Am 05.02.2015 um 17:35 schrieb riel@...hat.com:
>> 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, so 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       | 8 +++++++-
>>  include/linux/context_tracking_state.h | 1 +
>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
>> index bd9f000fc98d..a5d3bb44b897 100644
>> --- a/include/linux/context_tracking.h
>> +++ b/include/linux/context_tracking.h
>> @@ -43,7 +43,7 @@ static inline enum ctx_state exception_enter(void)
>>  static inline void exception_exit(enum ctx_state prev_ctx)
>>  {
>>  	if (context_tracking_is_enabled()) {
>> -		if (prev_ctx == IN_USER)
>> +		if (prev_ctx == IN_USER || prev_ctx == IN_GUEST)
>>  			context_tracking_user_enter(prev_ctx);
>>  	}
>>  }
>> @@ -78,6 +78,9 @@ static inline void guest_enter(void)
>>  		vtime_guest_enter(current);
>>  	else
>>  		current->flags |= PF_VCPU;
>> +
>> +	if (context_tracking_is_enabled())
>> +		context_tracking_user_enter(IN_GUEST);
>>  }
> 
> 
> Couldnt we make
>     rcu_virt_note_context_switch(smp_processor_id());
> conditional in include/linux/kvm_host.h (kvm_guest_enter)
> 
> e.g. something like
>     if (!context_tracking_is_enabled())
> 	    rcu_virt_note_context_switch(smp_processor_id());

Possibly. I considered the same, but I do not know whether
or not just rcu_user_enter / rcu_user_exit is enough.

I could certainly try it out and see whether anything
explodes, but I am not convinced that is careful enough
when it comes to handling RCU code...

Paul? :)

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