Re: [PATCH 5/5] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

Paul E. McKenney
2015-02-10T21:42:15Z

On Tue, Feb 10, 2015 at 03:27:54PM -0500, 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>

The checks for context_tracking_is_enabled() around the calls to
context_tracking_enter() and context_tracking_exit() (as opposed to
only within these functions) looked strange at first, but it avoids
a needless unconditional call in cases where the static_key disables
context tracking. (This is due to separate compilation.)

So...

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.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);
> }
>
> static inline void guest_exit(void)
> {
> + if (context_tracking_is_enabled())
> + context_tracking_exit(IN_GUEST);
> +
> if (vtime_accounting_enabled())
> vtime_guest_exit(current);
> else
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 72ab10fe1e46..90a7bab8779e 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -15,6 +15,7 @@ struct context_tracking {
> enum ctx_state {
> IN_KERNEL = 0,
> IN_USER,
> + IN_GUEST,
> } state;
> };
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 26f106022c88..c7828a6a9614 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -772,7 +772,8 @@ static inline void kvm_guest_enter(void)
> * one time slice). Lets treat guest mode as quiescent state, just like
> * we do with user-mode execution.
> */
> - rcu_virt_note_context_switch(smp_processor_id());
> + if (!context_tracking_cpu_is_enabled())
> + rcu_virt_note_context_switch(smp_processor_id());
> }
>
> static inline void kvm_guest_exit(void)
> --
> 1.9.3
>