On 7/21/22 15:59, Steven Rostedt wrote:
> On Thu, 21 Jul 2022 14:08:38 +0200
> Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> wrote:
>
>> On 7/20/22 22:06, Steven Rostedt wrote:
>>>> +/* \
>>>> + * da_monitor_enabled_##name - checks if the monitor is enabled \
>>>> + */ \
>>>> +static inline bool da_monitor_enabled_##name(void) \
>>>> +{ \
>>> Should we add a:
>>>
>>> smp_rmb();
>>>
>>> here? And then a smp_wmb() where these switches get updated?
>>>
>>
>> Makes sense.
>>
>> Should I also add the READ_ONCE/WRITE_ONCE? like
>>
>> smp_rmb()
>> READ_ONCE(var)
>>
>> WRITE_ONCE(var, value)
>> smp_wmb()
>
> I'm not sure the WRITE_ONCE() is necessary with the memory barriers.
> Because they should also prevent gcc from doing anything after that
> barrier. As Linus once stated, most cases WRITE_ONCE() is useless, but it's
> fine to keep more for annotation (as to pair with the READ_ONCE()) than for
> anything that is critical.
Ack, I can keep for annotation.
>>
>> for all these on/off knobs, or just the barriers?
>>
>>> I guess how critical is it that these turn off immediately after the switch
>>> is flipped?
>>
>> It is not critical to continue the execution of those that have already crossed by
>> the variable. Still, waiting for the tracepoints to finish their execution before
>> returning to the user-space task that disabled the variable might be a good thing.
>
> You mean after disabling, to wait for the tracepoints that are currently
> running to end?
yes, after disabling tracepoints.
>>
>> IIRC, we can do that via RCU... like, synchronize_rcu()?
>
> We have tracepoint_synchronize_unregister() that does that, as some
> traceponits use SRCU and not RCU.
yep, that is it!
-- Daniel
> -- Steve
>
>
>>
>>>> + /* global switch */ \
>>>> + if (unlikely(!rv_monitoring_on())) \
>>>> + return 0; \
>>>> + \
>>>> + /* monitor enabled */ \
>>>> + if (unlikely(!rv_##name.enabled)) \
>>>> + return 0; \
>>>> + \
>>>> + return 1; \
>>>> +} \
>>>> + \
>
