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

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> 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.

IMHO not 95%, but it is a start.

I would just like to avoid that too much efforts are spent on
re-inventing smart trace buffers, trace daemons, or trace visualization
tools. Then better pick up some semi-perfect approach (e.g. [1], it
unfortunately still seems to lack kernel integration) and drive it
according to our needs.

Jan

[1] http://lttng.org/ust

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