Re: [Qemu-devel] [PATCH] trace: only create simple backend trace files when an event is emitted.

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

Hollis Blanchard <hollis_blanchard@mentor.com> writes:

> Previously, trace record files (created by st_set_trace_file_enabled(true))
> were created by the simple backend at initialization time, even if no events
> were actually enabled.
>
> As a consequence, the working directory would become littered with trace-PID
> files just by enabling the simple backend at configure time, even when no
> -trace options were used.
>
> ---
> I've been using this to avoid creation of empty trace record files. I haven't
> done any scientific performance studies, but an extra function
> call/conditional/return when recording every trace event doesn't seem to have
> any noticeable effect.
>
> Signed-off-by: Hollis Blanchard <hollis_blanchard@mentor.com>
> ---
>  trace/simple.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/trace/simple.c b/trace/simple.c
> index 3fdcc82..67ccc3c 100644
> --- a/trace/simple.c
> +++ b/trace/simple.c
> @@ -209,6 +209,8 @@ int trace_record_start(TraceBufferRecord *rec, TraceEventID event, size_t datasi
>      uint64_t event_u64 = event;
>      uint64_t timestamp_ns = get_clock();
>
> +    st_set_trace_file_enabled(true);
> +
>      do {
>          old_idx = g_atomic_int_get(&trace_idx);
>          smp_rmb();
> @@ -320,6 +322,7 @@ void st_set_trace_file_enabled(bool enable)
>   */
>  void st_set_trace_file(const char *file)
>  {
> +    /* Tracing will be re-enabled when the next event is written. */
>      st_set_trace_file_enabled(false);
>
>      g_free(trace_file_name);
> @@ -330,8 +333,6 @@ void st_set_trace_file(const char *file)
>      } else {
>          trace_file_name = g_strdup_printf("%s", file);
>      }
> -
> -    st_set_trace_file_enabled(true);
>  }

This fixes the "make check" bug I reported:

http://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg01811.html

because by not enabling the file it doesn't attempt to flush it before
the test has started. However as it is a side-effect of the patch there
may be a better solution. However have a:

Tested-by: Alex Bennée <alex.bennee@linaro.org>

--
Alex Bennée