Re: [Qemu-devel] [PATCH 1/2] trace: Add simple tracing support

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/21/2010 08:46 AM, Jan Kiszka wrote:
> Anthony Liguori wrote:
>    
>> On 05/21/2010 04:42 AM, Stefan Hajnoczi wrote:
>>      
>>> Trace events should be defined in trace.h.  Events are written to
>>> /tmp/trace.log and can be formatted using trace.py.  Remember to add
>>> events to trace.py for pretty-printing.
>>>
>>> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>>> ---
>>>    Makefile.objs |    2 +-
>>>    trace.c       |   64
>>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>    trace.h       |    9 ++++++++
>>>    trace.py      |   30 ++++++++++++++++++++++++++
>>>    4 files changed, 104 insertions(+), 1 deletions(-)
>>>    create mode 100644 trace.c
>>>    create mode 100644 trace.h
>>>    create mode 100755 trace.py
>>>
>>> diff --git a/Makefile.objs b/Makefile.objs
>>> index acbaf22..307e989 100644
>>> --- a/Makefile.objs
>>> +++ b/Makefile.objs
>>> @@ -8,7 +8,7 @@ qobject-obj-y += qerror.o
>>>    # block-obj-y is code used by both qemu system emulation and qemu-img
>>>
>>>    block-obj-y = cutils.o cache-utils.o qemu-malloc.o qemu-option.o
>>> module.o
>>> -block-obj-y += nbd.o block.o aio.o aes.o osdep.o qemu-config.o
>>> +block-obj-y += nbd.o block.o aio.o aes.o osdep.o qemu-config.o trace.o
>>>    block-obj-$(CONFIG_POSIX) += posix-aio-compat.o
>>>    block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>>>
>>> diff --git a/trace.c b/trace.c
>>> new file mode 100644
>>> index 0000000..2fec4d3
>>> --- /dev/null
>>> +++ b/trace.c
>>> @@ -0,0 +1,64 @@
>>> +#include<stdlib.h>
>>> +#include<stdio.h>
>>> +#include "trace.h"
>>> +
>>> +typedef struct {
>>> +    unsigned long event;
>>> +    unsigned long x1;
>>> +    unsigned long x2;
>>> +    unsigned long x3;
>>> +    unsigned long x4;
>>> +    unsigned long x5;
>>> +} TraceRecord;
>>> +
>>> +enum {
>>> +    TRACE_BUF_LEN = 64 * 1024 / sizeof(TraceRecord),
>>> +};
>>> +
>>> +static TraceRecord trace_buf[TRACE_BUF_LEN];
>>> +static unsigned int trace_idx;
>>> +static FILE *trace_fp;
>>> +
>>> +static void trace(TraceEvent event, unsigned long x1,
>>> +                  unsigned long x2, unsigned long x3,
>>> +                  unsigned long x4, unsigned long x5) {
>>> +    TraceRecord *rec =&trace_buf[trace_idx];
>>> +    rec->event = event;
>>> +    rec->x1 = x1;
>>> +    rec->x2 = x2;
>>> +    rec->x3 = x3;
>>> +    rec->x4 = x4;
>>> +    rec->x5 = x5;
>>> +
>>> +    if (++trace_idx == TRACE_BUF_LEN) {
>>> +        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;
>>> +        }
>>> +    }
>>> +}
>>>
>>>        
>> It is probably worth while to read trace points via the monitor or
>> through some other mechanism.  My concern would be that writing even 64k
>> out to disk would introduce enough performance overhead mainly because
>> it runs lock-step with the guest's VCPU.
>>
>> Maybe it's worth adding a thread that syncs the ring to disk if we want
>> to write to disk?
>>      
> That's not what QEMU should worry about. If somehow possible, let's push
> this into the hands of a (user space) tracing framework, ideally one
> that is already designed for such requirements. E.g. there exists quite
> useful work in the context of LTTng (user space RCU for application
> tracing).
>    

 From what I understand, none of the current kernel approaches to 
userspace tracing have much momentum at the moment.

> We may need simple stubs for the case that no such framework is (yet)
> available. But effort should focus on a QEMU infrastructure to add
> useful tracepoints to the code. Specifically when tracing over KVM, you
> usually need information about kernel states as well, so you depend on
> an integrated approach, not Yet Another Log File.
>    

I think the simple code that Stefan pasted gives us 95% of what we need.

Regards,

Anthony Liguori

> Jan
>
>