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

[Lluís Vilanova]

Stefan Hajnoczi writes:

> 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().

Oh, I wasn't aware of the bitmap functions. Nice catch.


>> @@ -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?

Sorry, I have a tendency to make this type of checks explicit when the types are
not boolean (for a maybe-false sense of future-proofing). I can leave it as it
was if it bothers you.


>> +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.

Yes, it's a bit messy. I'll add some proper documentation about how this is
interpreted.


>> +
>> +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?

It just sounds proper to me to use size_t, since the state can never be negative
(it's either interpreted as a boolean or as an unsigned counter, depending on
the "vcpu" property).


Thanks,
  Lluis