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From: Rik van Riel <riel@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>, kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mtosatti@redhat.com,
	mingo@kernel.org, ak@linux.intel.com, oleg@redhat.com,
	masami.hiramatsu.pt@hitachi.com, fweisbec@gmail.com,
	paulmck@linux.vnet.ibm.com, lcapitulino@redhat.com,
	pbonzini@redhat.com
Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest
Date: Thu, 05 Feb 2015 11:52:37 -0500	[thread overview]
Message-ID: <54D39FD5.2040402@redhat.com> (raw)
In-Reply-To: <54D39DF0.3020109@de.ibm.com>

On 02/05/2015 11:44 AM, Christian Borntraeger wrote:
> Am 05.02.2015 um 17:35 schrieb riel@redhat.com:
>> From: Rik van Riel <riel@redhat.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@redhat.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? :)


  reply	other threads:[~2015-02-05 16:52 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05 16:35 [PATCH 0/4] rcu,nohz,kvm: " riel
2015-02-05 16:35 ` [PATCH 1/4] rcu,nohz: add state parameter to context_tracking_user_enter/exit riel
2015-02-05 16:35 ` [PATCH 2/4] rcu,nohz: run vtime_user_enter/exit only when state == IN_USER riel
2015-02-05 16:35 ` [PATCH 3/4] nohz,kvm: export context_tracking_user_enter/exit riel
2015-02-05 16:35 ` [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest riel
2015-02-05 16:44   ` Christian Borntraeger
2015-02-05 16:52     ` Rik van Riel [this message]
2015-02-05 17:50       ` Paul E. McKenney
2015-02-05 18:09         ` Rik van Riel
2015-02-05 18:56           ` Paul E. McKenney
2015-02-05 18:59             ` Rik van Riel
2015-02-05 19:02             ` Rik van Riel
2015-02-05 19:27               ` Paul E. McKenney
2015-02-05 20:19                 ` Rik van Riel

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