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SubjectRe: [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@linux.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@redhat.com wrote:
>> >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. 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 | 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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