[dtrace-discuss] When and when don't probe clauses record

[Jonathan Haslam]

Chip Bennett wrote On 10/23/06 16:08,:

> What I'm seeing is:
>
> fbt::clock:entry  { self->in = 1; }
>
> generates no recording data, but:
>
> fbt::clock:entry { self->in = 1; exit(0); }
>
> generates the default data (EPID).  Any action (as define by the doc, 
> which means an assignment statement is not an action) causes the 
> default action to happen.

I think the user guide can appear to be slightly ambiguous
here as to the description of exactly what an action is. Chapter
4 ( D Program and Structure) may not appear, at first, to
tally with the description given in Chapter 10 (Actions and
Subroutines). However, a DIF expression is indeed an action. It
may not be a data recording action but it is an action (of type
DTRACEACT_DIFEXPR - see sys/dtrace.h). Maybe one of the
team will chime in here if they have any differing view.

Your second example above contains a data recording action, exit().
It has to store data because it needs to return the exit status.
The default action is to record the EPID and nothing else but
here we record the EPID and the exit status. It looks the same
as the default action because the consumer (dtrace(1M)) is spitting
out its usual headers of CPU, probeid and probefunc/probename and
there is no additional ouput generated by the exit() action.

Hope this helps.

Cheers.

Jon.

>
> Chip
>
> Jonathan Haslam wrote:
>
>>
>>> I'm not seeing: 1) no actions, record nothing, 2) non-recording 
>>> actions, record default
>>
>>
>> I could be misunderstanding you here but just to clarify. What
>> you should be seeing here is this:
>>
>> If a clause contains *any* actions (any statements) at all then
>> we do not take the default action but store just what is explicitly
>> recorded. Therefore:
>>
>> fbt::clock:entry
>> {
>>    self->in = 1;
>> }
>>
>> stores nothing into the principal buffer whereas:
>>
>> fbt::clock:entry
>> {
>> }
>>
>> will record the EPID. If you are seeing any different to this then
>> let us know.
>>
>> Cheers.
>>
>> Jon.
>>
>>>
>>> But thanks for the confirmation.
>>>
>>> At the same time, this makes sense.  If all you're doing is making 
>>> calculations (like future predicates, or aggregations), you probably 
>>> don't want to see that the probe fired.
>>>
>>> Chip
>>>
>>> Jonathan Haslam wrote:
>>>
>>>> Hi Chip,
>>>>
>>>> Your understanding is indeed correct. If a clause contains actions
>>>> and none of them store any data then we don't even record the EPID
>>>> into the principal buffer.
>>>>
>>>> The DTrace user guide implicitly states this in the discussion at the
>>>> start of Chapter 10 on the "Default Action".
>>>>
>>>> Cheers.
>>>>
>>>> Jon.
>>>>
>>>>
>>>> Chip Bennett wrote On 10/21/06 20:10,:
>>>>
>>>>> I've noticed in the past that probe clauses don't always record 
>>>>> into the buffer.  I don't see anything in the manual that 
>>>>> specifically states this, but it appears that if a probe clause 
>>>>> has statements (i.e. assignments), but no actions (recording or 
>>>>> exit(), etc.) that the probe clause doesn't even record the EPID.  
>>>>> Can anyone confirm this to be an accurate assessment?  Is there 
>>>>> more to the rule of when a clause records and when it doesn't, or 
>>>>> is this pretty much it?
>>>>>
>>>>> (Making sure my info is accurate for an article.)
>>>>>
>>>>> Thanks,
>>>>> Chip
>>>>> _______________________________________________
>>>>> dtrace-discuss mailing list
>>>>> dtrace-discuss at opensolaris.org
>>>>
>>>>
>>>>
>>>
>>> _______________________________________________
>>> dtrace-discuss mailing list
>>> dtrace-discuss at opensolaris.org
>>
>>
>