[DTrace-devel] semantics of "timestamp"

[Eugene Loh]

On 07/16/2020 01:16 PM, Kris Van Hees wrote:

> On Thu, Jul 16, 2020 at 12:57:17PM -0700, Eugene Loh wrote:
>> What are the semantics of the built-in variable "timestamp"?
>> Specifically, does it record a fresh time stamp each time it is called?
>> Or it is measured once upon probe (or clause) entry and then reused?
>>
>> The documentation isn't real specific, but it seems to imply a fresh
>> time on each call.  Meanwhile, DTv1 seems to get a fresh call for each
>> clause and then reuse that value during that clause.  In contrast, DTv2
>> seems to get a fresh value with each clause.  Is the change from DTv1 to
>> DTv2 intentional?  Should the legacy behavior be preserved?
> The correct behaviour based on the reference implementation is that the value
> of 'timestamp' is consistent within the scope of a clause (and its predicate).
> Therefore, yes, we need to fix this.
>
> What needs to happen is that the mstate structure needs to get a new field
> to hold the timestamp value, and it should be initialized to 0 in the
> prologue code generated in dt_cg_prologue().  Then, in get_bvar() we need to
> check to see if mst->timestamp is 0.  If it is, we use the BPF helper to obtain
> the timestamp value and we store that in mst->timestamp.  Either way, we then
> return mst->timestamp as value.
>
> That will ensure that the value is consistent for multiple timestamp references
> within a clause (and its predicate) while ensuring that between clauses we get
> a new value.
>
> Want to tackle that?
Sure.  Will finish raise() first.  It's basically done, but the small 
stuff and the unrelated bugs are slowing me down.