On Thu, Feb 05, 2015 at 11:52:37AM -0500, Rik van Riel wrote:
> On 02/05/2015 11:44 AM, Christian Borntraeger wrote:
> > Am 05.02.2015 um 17:35 schrieb riel@xxxxxxxxxx:
> >> From: Rik van Riel <riel@xxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxx>
> >> ---
> >> 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? :)
That can fail for some odd combinations of Kconfig and boot parameters.
As I understand it, you instead want the following:
if (!tick_nohz_full_cpu(smp_processor_id()))
rcu_virt_note_context_switch(smp_processor_id());
Frederic might know of a better approach.
Thanx, Paul
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