Re: [PATCH v2 3/3] xen: support runqueue steal time on xen

[Stefano Stabellini]

On Wed, 6 Jul 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 

Reviewed-by: Stefano Stabellini 


> V2: removed static variable as requested by Stefano Stabellini
> ---
>  drivers/xen/time.c | 40 +++-
>  1 file changed, 23 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index 2257b66..a7fe35b 100644
> --- a/drivers/xen/time.c
> +++ b/drivers/xen/time.c
> @@ -47,27 +47,31 @@ 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(_runstate);
> + state = per_cpu_ptr(_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_entry_time);
> + rmb();  /* Hypervisor might update data. */
>   *res = READ_ONCE(*state);
> - } while (get64(>state_entry_time) != state_time);
> + rmb();  /* Hypervisor might update data. */
> + } while (get64(>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 +84,7 @@ static u64 xen_steal_clock(int cpu)
>  {
>   struct vcpu_runstate_info state;
>  
> - BUG_ON(cpu != smp_processor_id());
> - xen_get_runstate_snapshot();
> + xen_get_runstate_snapshot_cpu(, cpu);
>   return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline];
>  }
>  
> @@ -98,11 +101,14 @@ void xen_setup_runstate_info(int cpu)
>  
>  void __init xen_time_setup_guest(void)
>  {
> + bool xen_runstate_remote;
> +
> + 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(_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(_steal_rq_enabled);
>  }
> -- 
> 2.6.6
> 

Re: [PATCH v2 3/3] xen: support runqueue steal time on xen

[Stefano Stabellini]

On Wed, 6 Jul 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 

Reviewed-by: Stefano Stabellini 


> V2: removed static variable as requested by Stefano Stabellini
> ---
>  drivers/xen/time.c | 40 +++-
>  1 file changed, 23 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/xen/time.c b/drivers/xen/time.c
> index 2257b66..a7fe35b 100644
> --- a/drivers/xen/time.c
> +++ b/drivers/xen/time.c
> @@ -47,27 +47,31 @@ 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(_runstate);
> + state = per_cpu_ptr(_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_entry_time);
> + rmb();  /* Hypervisor might update data. */
>   *res = READ_ONCE(*state);
> - } while (get64(>state_entry_time) != state_time);
> + rmb();  /* Hypervisor might update data. */
> + } while (get64(>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 +84,7 @@ static u64 xen_steal_clock(int cpu)
>  {
>   struct vcpu_runstate_info state;
>  
> - BUG_ON(cpu != smp_processor_id());
> - xen_get_runstate_snapshot();
> + xen_get_runstate_snapshot_cpu(, cpu);
>   return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline];
>  }
>  
> @@ -98,11 +101,14 @@ void xen_setup_runstate_info(int cpu)
>  
>  void __init xen_time_setup_guest(void)
>  {
> + bool xen_runstate_remote;
> +
> + 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(_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(_steal_rq_enabled);
>  }
> -- 
> 2.6.6
>