You may be able to combine Jonathan's link-following method with a tick probe
to iterate over an arbitrarily long list over time. There's a chance the list
can change out from under you, but it should get close to what you want.
my:probe:matching:func
{
this->list = /* some way to get initial list */;
this->length = 0;
}
:::tick-5000 / this->list != NULL / {
this->list = this->list->next; this->length++;
}
:::tick-5000 / this->list == NULL / {
/* use this->count as the length of the list */
trace(this->length);
exit(0);
}
I'm not sure how to verify that the tick probe has fired in the appropriate
"this" context. If tick fires in the wrong context, you could use any high
frequency probe in the right context (fbt::: ?) to accomplish the same thing.
Kevin
On Aug 19, 2011, at 3:43 PM, Lida Horn <lida.h...@oracle.com> wrote:
> Thank you for your reply, however I had
> realized I could do something like that, but it a
> stanza per iteration of the loop and if the loop
> can be large (in the case I was looking at well over
> 4000 entries) the number of stanzas is prohibitive.
>
> I'm looking for a way to actually follow the chain
> all the way to the end without a hard limit
> and without a huge dtrace script.
>
> Regards,
> Lida Horn
>
>
> requires On 8/19/2011 2:46 PM, Jonathan Adams wrote:
>> On Fri, Aug 19, 2011 at 01:34:36PM -0700, Lida Horn wrote:
>>> I'm looking for an example of how one could write a dtrace probe
>>> that could follow something like a NULL terminated linked list.
>>>
>>> For example:
>>>
>>> struct list {
>>> struct list *next;
>>> void *data;
>>> };
>>>
>>> struct list *list_root;
>>>
>>>
>>> Assuming I had "list_root" available, but I wanted to know how long
>>> the linked list is, how can I do this in a dtrace probe?
>>>
>>> If I had looping constructs, it would be obvious:
>>>
>>> int
>>> count_list(struct list *list)
>>> {
>>> int i;
>>>
>>> for (i = 0; list != NULL; list = list->next)
>>> ++i;
>>>
>>> return i;
>>> }
>>>
>>> In the dtrace scripting language there are no loop constructs, but is
>>> there a way to add a new provider than can loop?
>> Generally, you can do this using something like:
>>
>> my:probe:matching:func
>> {
>> this->list = /* some way to get initial list */;
>> this->length = 0;
>> }
>>
>> my:probe:matching:func / this->list != NULL / {
>> this->list = this->list->next; this->length++;
>> }
>> my:probe:matching:func / this->list != NULL / {
>> this->list = this->list->next; this->length++;
>> }
>> my:probe:matching:func / this->list != NULL / {
>> this->list = this->list->next; this->length++;
>> }
>> my:probe:matching:func / this->list != NULL / {
>> this->list = this->list->next; this->length++;
>> }
>> /* repeat N times, where N is the max list length */
>>
>> my:probe:matching:func / this->list != NULL / {
>> /* list was longer than our maximum length */
>> @overflow = count();
>> }
>> my:probe:matching:func / this->list == NULL / {
>> /* use this->count as the length of the list */
>> }
>>
>>
>> (Enablings for a probe are guaranteed to execute in program order, and this->
>> variables are valid between enablings of the same probe.)
>>
>> Cheers,
>> - jonathan
>>
>>
>
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