Re: [Qemu-devel] [PATCH 3/3] trace: Flush trace buffer on exit

[Prerna Saxena <prerna@linux.vnet.ibm.com>]

Hi Stefan,
On 07/07/2010 01:44 AM, Stefan Hajnoczi wrote:
> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
> ---
> This applies to the tracing branch at:
>
>    http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/tracing-dev
>
>   simpletrace.c |   23 +++++++++++++++--------
>   1 files changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/simpletrace.c b/simpletrace.c
> index ace009f..9604ea6 100644
> --- a/simpletrace.c
> +++ b/simpletrace.c
> @@ -22,6 +22,20 @@ static TraceRecord trace_buf[TRACE_BUF_LEN];
>   static unsigned int trace_idx;
>   static FILE *trace_fp;
>
> +static void flush_trace_buffer(void)
> +{
> +    if (!trace_fp) {
> +        trace_fp = fopen("/tmp/trace.log", "w");
> +        if (trace_fp) {
> +            atexit(flush_trace_buffer);
> +        }
> +    }
> +    if (trace_fp) {
> +        size_t unused; /* for when fwrite(3) is declared warn_unused_result */
> +        unused = fwrite(trace_buf, trace_idx * sizeof(trace_buf[0]), 1, trace_fp);

I think this would be better denoted as :
unused = fwrite(trace_buf, trace_idx * sizeof(TraceRecord), 1, trace_fp);

> +    }
> +}
> +
>   static void trace(TraceEventID event, unsigned long x1,
>                     unsigned long x2, unsigned long x3,
>                     unsigned long x4, unsigned long x5)
> @@ -44,15 +58,8 @@ static void trace(TraceEventID event, unsigned long x1,
>       rec->x5 = x5;
>
>       if (++trace_idx == TRACE_BUF_LEN) {
> +        flush_trace_buffer();
>           trace_idx = 0;
> -
> -        if (!trace_fp) {
> -            trace_fp = fopen("/tmp/trace.log", "w");
> -        }
> -        if (trace_fp) {
> -            size_t result = fwrite(trace_buf, sizeof trace_buf, 1, trace_fp);
> -            result = result;
> -        }
>       }
>   }
>

I was wondering if we can extend this. One can have a monitor command 
such as "dump-trace" which would write a partly-filled buffer to file 
using a call to flush_trace_buffer().
But this has a few caveats. flush_trace_buffer() must reset trace_idx to 
0 to prevent duplicate traces to be written once the buffer is filled up.
Also, I'm wondering what happens in case qemu is started with -smp 2 or 
more. We might need to enforce some kind of synchronisation so that 
threads on other cpus do not log traces while the buffer is being 
sync'ed. ( For now, I have not been able to get upstream qemu run with 
-smp. Going forward, this is something that might need to be looked into.)

Regards,

-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India