Re: [Qemu-devel] [PATCH 4/6] trace: Add per-vCPU tracing states for events with the 'vcpu' property

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Feb 25, 2016 at 04:03:06PM +0100, Lluís Vilanova wrote:
 @@ -332,6 +334,16 @@ struct CPUState {
>      struct KVMState *kvm_state;
>      struct kvm_run *kvm_run;
>  
> +#define TRACE_VCPU_DSTATE_TYPE uint32_t
> +    TRACE_VCPU_DSTATE_TYPE trace_dstate;

Please use typedef instead of #define.

> +    /*
> +     * Ensure 'trace_dstate' can encode event states as a bitmask. This limits
> +     * the number of events with the 'vcpu' property and *without* the
> +     * 'disabled' property.
> +     */
> +    bool too_many_vcpu_events[
> +        TRACE_VCPU_EVENT_COUNT > sizeof(TRACE_VCPU_DSTATE_TYPE)*8 ? -1 : 0];

Why limit ourselves to a scalar when "qemu/bitops.h" and "qemu/bitmap.h"
provide functions for arbitrary length bitmaps?

See DECLARE_BITMAP(), set_bit(), clear_bit(), test_bit().

> @@ -61,7 +69,7 @@ static inline bool trace_event_get_state_static(TraceEvent *ev)
>  static inline bool trace_event_get_state_dynamic_by_id(TraceEventID id)
>  {
>      /* it's on fast path, avoid consistency checks (asserts) */
> -    return unlikely(trace_events_enabled_count) && trace_events_dstate[id];
> +    return unlikely(trace_events_enabled_count) && (trace_events_dstate[id] > 0);

typeof(trace_events_dstate[0]) is size_t, so trace_events_dstate[id] > 0
is equivalent to trace_events_dstate[id] (due to unsigned).  Why change
this line?

> +void trace_event_set_state_dynamic(TraceEvent *ev, bool state)
> +{
> +    CPUState *cpu;
> +    assert(ev != NULL);
> +    assert(trace_event_get_state_static(ev));
> +    if (trace_event_get_cpu_id(ev) != trace_event_cpu_count()) {
> +        CPU_FOREACH(cpu) {
> +            trace_event_set_cpu_state_dynamic(cpu, ev, state);
> +        }
> +    } else {
> +        TraceEventID id = trace_event_get_id(ev);
> +        trace_events_enabled_count += state - trace_events_dstate[id];
> +        trace_events_dstate[id] = state;
> +    }
> +}

I find it a little confusing to use different semantics for
trace_events_dstate[] elements depending on trace_event_get_cpu_id(ev)
!= trace_event_cpu_count().  In other words, it either acts as a vcpu
enabled counter or as an enable/disable flag.

That said, it's nice to preserve the non-cpu_id case since it was
written by Paolo as a performance optimization.  Changing it could
introduce a regression so I think your approach is okay.

> +
> +void trace_event_set_cpu_state_dynamic(CPUState *cpu,
> +                                       TraceEvent *ev, bool state)
> +{
> +    TraceEventID id;
> +    TraceEventVCPUID cpu_id;
> +    TRACE_VCPU_DSTATE_TYPE bit;
> +    bool state_pre;
> +    assert(cpu != NULL);
> +    assert(ev != NULL);
> +    assert(trace_event_get_state_static(ev));
> +    assert(trace_event_get_cpu_id(ev) != trace_event_cpu_count());
> +    id = trace_event_get_id(ev);
> +    cpu_id = trace_event_get_cpu_id(ev);
> +    bit = ((TRACE_VCPU_DSTATE_TYPE)1) << cpu_id;
> +    state_pre = cpu->trace_dstate & bit;
> +    if ((state_pre == 0) != (state == 0)) {

Simpler expression:

if (state_pre != state)

> @@ -21,7 +21,10 @@
>  #include "qemu/error-report.h"
>  
>  int trace_events_enabled_count;
> -bool trace_events_dstate[TRACE_EVENT_COUNT];
> +/* With the 'vcpu' property, counts how many vCPUs have it enabled. */
> +size_t trace_events_dstate[TRACE_EVENT_COUNT];

The number of cpus has type int (see CPUState *qemu_get_cpu(int index)).

Why did you choose size_t?

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