Re: [Xen-devel] [PATCH 3/3] xen: support runqueue steal time on xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 22 Jun 2016, Juergen Gross wrote:
> Up to now reading the stolen time of a remote cpu was not possible in a
> performant way under Xen. This made support of runqueue steal time via
> paravirt_steal_rq_enabled impossible.
> 
> With the addition of an appropriate hypervisor interface this is now
> possible, so add the support.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  drivers/xen/time.c | 42 +++++++++++++++++++++++++-----------------
>  1 file changed, 25 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index 2257b66..65afbe9 100644
> --- a/drivers/xen/time.c
> +++ b/drivers/xen/time.c
> @@ -19,6 +19,9 @@
>  /* runstate info updated by Xen */
>  static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate);
>  
> +/* runstate info of remote cpu accessible */
> +static bool xen_runstate_remote;

Honestly I would rather have one global variable less if it means only
dropping one BUG_ON.


>  /* return an consistent snapshot of 64-bit time/counter value */
>  static u64 get64(const u64 *p)
>  {
> @@ -47,27 +50,31 @@ static u64 get64(const u64 *p)
>       return ret;
>  }
>  
> -/*
> - * Runstate accounting
> - */
> -void xen_get_runstate_snapshot(struct vcpu_runstate_info *res)
> +static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res,
> +                                       unsigned int cpu)
>  {
>       u64 state_time;
>       struct vcpu_runstate_info *state;
>  
>       BUG_ON(preemptible());
>  
> -     state = this_cpu_ptr(&xen_runstate);
> +     state = per_cpu_ptr(&xen_runstate, cpu);
>  
> -     /*
> -      * The runstate info is always updated by the hypervisor on
> -      * the current CPU, so there's no need to use anything
> -      * stronger than a compiler barrier when fetching it.
> -      */
>       do {
>               state_time = get64(&state->state_entry_time);
> +             rmb();  /* Hypervisor might update data. */
>               *res = READ_ONCE(*state);
> -     } while (get64(&state->state_entry_time) != state_time);
> +             rmb();  /* Hypervisor might update data. */
> +     } while (get64(&state->state_entry_time) != state_time ||
> +              (state_time & XEN_RUNSTATE_UPDATE));
> +}
> +
> +/*
> + * Runstate accounting
> + */
> +void xen_get_runstate_snapshot(struct vcpu_runstate_info *res)
> +{
> +     xen_get_runstate_snapshot_cpu(res, smp_processor_id());
>  }
>  
>  /* return true when a vcpu could run but has no real cpu to run on */
> @@ -80,8 +87,8 @@ static u64 xen_steal_clock(int cpu)
>  {
>       struct vcpu_runstate_info state;
>  
> -     BUG_ON(cpu != smp_processor_id());
> -     xen_get_runstate_snapshot(&state);
> +     BUG_ON(!xen_runstate_remote && cpu != smp_processor_id());
> +     xen_get_runstate_snapshot_cpu(&state, cpu);
>       return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline];
>  }
>  
> @@ -98,11 +105,12 @@ void xen_setup_runstate_info(int cpu)
>  
>  void __init xen_time_setup_guest(void)
>  {
> +     xen_runstate_remote = !HYPERVISOR_vm_assist(VMASST_CMD_enable,
> +                                     VMASST_TYPE_runstate_update_flag);
> +
>       pv_time_ops.steal_clock = xen_steal_clock;
>  
>       static_key_slow_inc(&paravirt_steal_enabled);
> -     /*
> -      * We can't set paravirt_steal_rq_enabled as this would require the
> -      * capability to read another cpu's runstate info.
> -      */
> +     if (xen_runstate_remote)
> +             static_key_slow_inc(&paravirt_steal_rq_enabled);
>  }

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