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 12:19:28 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>, Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	kvm list <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when
 running KVM guest

On Tue, Feb 10, 2015 at 12:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, Feb 10, 2015 at 11:59:09AM -0800, 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.  (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.)
>
> It turns out that context_tracking_is_enabled() is a static inline
> that uses a static_key, so the overhead should be minimal on platforms
> having a full implementation of static keys.

Shouldn't we just fold that into context_tracking_xyz_enter?

Also, why does the vtime stuff depend on RCU extended quiescent
states?  To me, they seem mostly orthogonal other than the fact that
they hook into the same places.

--Andy
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