Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch

[Frederic Weisbecker]

(Adding  Christian Borntraeger in Cc)

2012/10/25 Frederic Weisbecker <fweisbec@xxxxxxxxx>:
> Switching to or from guest context is done on ioctl context.
> So by the time we call kvm_guest_enter() or kvm_guest_exit()
> we know we are not running the idle task.
>
> As a result, we can directly account the cputime using
> vtime_account_system().
>
> There are two good reasons to do this:
>
> * We avoid some useless checks on guest switch. It optimizes
> a bit this fast path.
>
> * In the case of CONFIG_IRQ_TIME_ACCOUNTING, calling vtime_account()
> checks for irq time to account. This is pointless since we know
> we are not in an irq on guest switch. This is wasting cpu cycles
> for no good reason. vtime_account_system() OTOH is a no-op in
> this config option.
>
> * We can remove the irq disable/enable around kvm guest switch in s390.
>
> A further optimization may consist in introducing a vtime_account_guest()
> that directly calls account_guest_time().
>
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Cc: Avi Kivity <avi@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Joerg Roedel <joerg.roedel@xxxxxxx>
> Cc: Alexander Graf <agraf@xxxxxxx>
> Cc: Xiantao Zhang <xiantao.zhang@xxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> ---
>  arch/s390/kvm/kvm-s390.c |    4 ----
>  include/linux/kvm_host.h |   12 ++++++++++--
>  2 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index ecced9d..d91a955 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -608,9 +608,7 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
>                 kvm_s390_deliver_pending_interrupts(vcpu);
>
>         vcpu->arch.sie_block->icptcode = 0;
> -       local_irq_disable();
>         kvm_guest_enter();
> -       local_irq_enable();
>         VCPU_EVENT(vcpu, 6, "entering sie flags %x",
>                    atomic_read(&vcpu->arch.sie_block->cpuflags));
>         trace_kvm_s390_sie_enter(vcpu,
> @@ -629,9 +627,7 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
>         VCPU_EVENT(vcpu, 6, "exit sie icptcode %d",
>                    vcpu->arch.sie_block->icptcode);
>         trace_kvm_s390_sie_exit(vcpu, vcpu->arch.sie_block->icptcode);
> -       local_irq_disable();
>         kvm_guest_exit();
> -       local_irq_enable();
>
>         memcpy(&vcpu->run->s.regs.gprs[14], &vcpu->arch.sie_block->gg14, 16);
>         return rc;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 93bfc9f..0e2212f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -737,7 +737,11 @@ static inline int kvm_deassign_device(struct kvm *kvm,
>  static inline void kvm_guest_enter(void)
>  {
>         BUG_ON(preemptible());
> -       vtime_account(current);
> +       /*
> +        * This is running in ioctl context so we can avoid
> +        * the call to vtime_account() with its unnecessary idle check.
> +        */
> +       vtime_account_system(current);
>         current->flags |= PF_VCPU;
>         /* KVM does not hold any references to rcu protected data when it
>          * switches CPU into a guest mode. In fact switching to a guest mode
> @@ -751,7 +755,11 @@ static inline void kvm_guest_enter(void)
>
>  static inline void kvm_guest_exit(void)
>  {
> -       vtime_account(current);
> +       /*
> +        * This is running in ioctl context so we can avoid
> +        * the call to vtime_account() with its unnecessary idle check.
> +        */
> +       vtime_account_system(current);
>         current->flags &= ~PF_VCPU;
>  }
>
> --
> 1.7.5.4
>
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