Re: [Qemu-devel] [PATCH v8 2/7] cpu: allocate cpu->trace_dstate in place

[Lluís Vilanova]

Emilio G Cota writes:

> There's little point in dynamically allocating the bitmap if we
> know at compile-time the max number of events we want to support.
> Thus, make room in the struct for the bitmap, which will make things
> easier later: this paves the way for upcoming changes, in which
> we'll use a u32 to fully capture cpu->trace_dstate.

> This change also increases performance by saving a dereference and
> improving locality--note that this is important since upcoming work
> makes reading this bitmap fairly common.

> Signed-off-by: Emilio G. Cota <address@hidden>

Reviewed-by: Lluís Vilanova <address@hidden>


> ---
>  include/qom/cpu.h | 9 +++------
>  qom/cpu.c         | 8 --------
>  trace/control.c   | 9 ++++++++-
>  3 files changed, 11 insertions(+), 15 deletions(-)

> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 89ddb68..bc6e20f 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -259,6 +259,7 @@ typedef void (*run_on_cpu_func)(CPUState *cpu, 
> run_on_cpu_data data);
>  struct qemu_work_item;
 
>  #define CPU_UNSET_NUMA_NODE_ID -1
> +#define CPU_TRACE_DSTATE_MAX_EVENTS 32
 
>  /**
>   * CPUState:
> @@ -373,12 +374,8 @@ struct CPUState {
>      struct KVMState *kvm_state;
>      struct kvm_run *kvm_run;
 
> -    /*
> -     * Used for events with 'vcpu' and *without* the 'disabled' properties.
> -     * Dynamically allocated based on bitmap requried to hold up to
> -     * trace_get_vcpu_event_count() entries.
> -     */
> -    unsigned long *trace_dstate;
> +    /* Used for events with 'vcpu' and *without* the 'disabled' properties */
> +    DECLARE_BITMAP(trace_dstate, CPU_TRACE_DSTATE_MAX_EVENTS);
 
>      /* TODO Move common fields from CPUArchState here. */
>      int cpu_index; /* used by alpha TCG */
> diff --git a/qom/cpu.c b/qom/cpu.c
> index 5069876..69fbb9c 100644
> --- a/qom/cpu.c
> +++ b/qom/cpu.c
> @@ -382,7 +382,6 @@ static void cpu_common_unrealizefn(DeviceState *dev, 
> Error **errp)
 
>  static void cpu_common_initfn(Object *obj)
>  {
> -    uint32_t count;
>      CPUState *cpu = CPU(obj);
>      CPUClass *cc = CPU_GET_CLASS(obj);
 
> @@ -397,18 +396,11 @@ static void cpu_common_initfn(Object *obj)
>      QTAILQ_INIT(&cpu->breakpoints);
>      QTAILQ_INIT(&cpu->watchpoints);
 
> -    count = trace_get_vcpu_event_count();
> -    if (count) {
> -        cpu->trace_dstate = bitmap_new(count);
> -    }
> -
>      cpu_exec_initfn(cpu);
>  }
 
>  static void cpu_common_finalize(Object *obj)
>  {
> -    CPUState *cpu = CPU(obj);
> -    g_free(cpu->trace_dstate);
>  }
 
>  static int64_t cpu_common_get_arch_id(CPUState *cpu)
> diff --git a/trace/control.c b/trace/control.c
> index 9b157b0..83740aa 100644
> --- a/trace/control.c
> +++ b/trace/control.c
> @@ -65,8 +65,15 @@ void trace_event_register_group(TraceEvent **events)
>      size_t i;
>      for (i = 0; events[i] != NULL; i++) {
>          events[i]->id = next_id++;
> -        if (events[i]->vcpu_id != TRACE_VCPU_EVENT_NONE) {
> +        if (events[i]->vcpu_id == TRACE_VCPU_EVENT_NONE) {
> +            continue;
> +        }
> +
> +        if (likely(next_vcpu_id < CPU_TRACE_DSTATE_MAX_EVENTS)) {
>              events[i]->vcpu_id = next_vcpu_id++;
> +        } else {
> +            error_report("WARNING: too many vcpu trace events; dropping 
> '%s'",
> +                         events[i]->name);
>          }
>      }
>      event_groups = g_renew(TraceEventGroup, event_groups, nevent_groups + 1);
> -- 
> 2.7.4