----- On Apr 25, 2018, at 6:51 PM, rostedt rostedt@xxxxxxxxxxx wrote:
> On Wed, 25 Apr 2018 17:40:56 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
>> One problem with your approach is that you can have multiple callers
>> for the same tracepoint name, where some could be non-preemptible and
>> others blocking. Also, there is then no clear way for the callback
>> registration API to enforce whether the callback expects the tracepoint
>> to be blocking or non-preemptible. This can introduce hard to diagnose
>> issues in a kernel without debug options enabled.
>
> I agree that it should not be tied to an implementation name. But
> "blocking" is confusing. I would say "can_sleep" or some such name that
> states that the trace point caller is indeed something that can sleep.
"trace_*event*_{can,might,may}_sleep" are all acceptable candidates for
me.
>
>>
>> Regarding the name, I'm OK with having something along the lines of
>> trace_*event*_blocking or such. Please don't use "srcu" or other naming
>> that is explicitly tied to the underlying mechanism used internally
>> however: what we want to convey is that this specific tracepoint probe
>> can be preempted and block. The underlying implementation could move to
>> a different RCU flavor brand in the future, and it should not impact
>> users of the tracepoint APIs.
>>
>> In order to ensure that probes that may block only register themselves
>> to tracepoints that allow blocking, we should introduce new tracepoint
>> declaration/definition *and* registration APIs also contain the
>> "BLOCKING/blocking" keywords (or such), so we can ensure that a
>> tracepoint probe being registered to a "blocking" tracepoint is indeed
>> allowed to block.
>
> I'd really don't want to add more declaration/definitions, as we
> already have too many as is, and with different meanings and the number
> is of incarnations is n! in growth.
>
> I'd say we just stick with a trace_<event>_can_sleep() call, and make
> sure that if that is used that no trace_<event>() call is also used, and
> enforce this with linker or compiler tricks.
My main concern is not about having both trace_<event>_can_sleep() mixed
with trace_<event>() calls. It's more about having a registration API allowing
modules registering probes that may need to sleep to explicitly declare it,
and enforce that tracepoint never connects a probe that may need to sleep
with an instrumentation site which cannot sleep.
I'm unsure what's the best way to achieve this goal though. We could possibly
extend the tracepoint_probe_register_* APIs to introduce e.g.
tracepoint_probe_register_prio_flags() and provide a TRACEPOINT_PROBE_CAN_SLEEP
as parameter upon registration. If this flag is provided, then we could figure out
an way to iterate on all callers, and ensure they are all "can_sleep" type of
callers.
Thoughts ?
Thanks,
Mathieu
>
> -- Steve
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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