Re: [PATCH v4 4/7] contrib/plugins/uftrace: add timestamp-based-on-real-time option

["Alex Bennée" <alex.bennee@linaro.org>]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> usage: timestamp-based-on-real-time=[on|off]
>
> Instead of using number of instructions executed (which is per vcpu), we
> use the wall time for timestamps. This is useful when tracing user mode
> programs as well.

Arguably it is the more useful default when examining flame graphs. The
instruction count is entirely artificial and wouldn't map onto real
execution time anyway.

Maybe use-icount-for-ts and default to off?

>
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> ---
>  contrib/plugins/uftrace.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/contrib/plugins/uftrace.c b/contrib/plugins/uftrace.c
> index bd4219db693..9211f273700 100644
> --- a/contrib/plugins/uftrace.c
> +++ b/contrib/plugins/uftrace.c
> @@ -94,8 +94,28 @@ enum uftrace_record_type {
>  static struct qemu_plugin_scoreboard *score;
>  static uint64_t trace_sample;
>  static bool trace_privilege_level;
> +static bool timestamp_based_on_real_time;
>  static CpuOps arch_ops;
>  
> +static uint64_t gettime_ns(void)
> +{
> +#ifdef _WIN32
> +    /*
> +     * On Windows, timespec_get is available only with UCRT, but not with
> +     * MinGW64 environment. Simplify by using only gettimeofday on this
> +     * platform.
> +     */
> +    struct timeval tv;
> +    gettimeofday(&tv, NULL);
> +    uint64_t now_ns = tv.tv_sec * 1000 * 1000 * 1000 + tv.tv_usec * 1000;
> +#else
> +    struct timespec ts;
> +    timespec_get(&ts, TIME_UTC);
> +    uint64_t now_ns = ts.tv_sec * 1000 * 1000 * 1000 + ts.tv_nsec;

Why use timspec_get here, QEMU's own timer uses:

  /* get host real time in nanosecond */
  static inline int64_t get_clock_realtime(void)
  {
      struct timeval tv;

      gettimeofday(&tv, NULL);
      return tv.tv_sec * 1000000000LL + (tv.tv_usec * 1000);
  }


> +#endif
> +    return now_ns;
> +}
> +
>  static void uftrace_write_map(bool system_emulation)
>  {
>      const char *path = "./uftrace.data/sid-0.map";
> @@ -454,6 +474,9 @@ static void cpu_set_new_sample(Cpu *cpu, uint64_t timestamp)
>  
>  static uint64_t cpu_get_timestamp(const Cpu *cpu)
>  {
> +    if (timestamp_based_on_real_time) {
> +        return gettime_ns();
> +    }
>      return cpu->insn_count;
>  }
>  
> @@ -866,6 +889,12 @@ QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
>                  fprintf(stderr, "boolean argument parsing failed: %s\n", opt);
>                  return -1;
>              }
> +        } else if (g_strcmp0(tokens[0], "timestamp-based-on-real-time") == 0) {
> +            if (!qemu_plugin_bool_parse(tokens[0], tokens[1],
> +                                        &timestamp_based_on_real_time)) {
> +                fprintf(stderr, "boolean argument parsing failed: %s\n", opt);
> +                return -1;
> +            }
>          } else {
>              fprintf(stderr, "option parsing failed: %s\n", opt);
>              return -1;

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro