[lttng-dev] URCU_CALL_RCU_RT and its implications

[Mark E. Dawson, Jr.]

And yes, the rcu_quiescent_state() is being regularly called in the updater thread, just as an fyi.

      From: "Mark E. Dawson, Jr. via lttng-dev" <lttng-dev at lists.lttng.org>
 To: Lttng-dev <lttng-dev at lists.lttng.org> 
 Sent: Thursday, December 8, 2016 1:53 PM
 Subject: [lttng-dev] URCU_CALL_RCU_RT and its implications
   
All,
In the documentation for the create_call_rcu_data() call, we have the following description of the URCU_CALL_RCU_RT flag:

struct call_rcu_data *create_call_rcu_data(unsigned long flags, int cpu_affinity): Returns a handle that can be passed to the other functions in this list. The flags argument can be zero, or can be URCU_CALL_RCU_RT if the application threads associated with the new callback-invocation thread are to get real-time response from call_rcu() by avoiding the need to call into the kernel to wake up the callback-invocation thread.

So setting "flags" to 0 will simply use the futex mechanism for wakeups. What exactly happens when set to the RT option? Is there something else that needs to be configured from the OS side to make the aforementioned option work correctly?
I'm asking because we're seeing intermittent issues whereby on application startup, the reclamation thread (call_rcu) does exactly one cleanup, but then never does it again.  We have the URCU_CALL_RCU_RT flag set.  Originally, we thought it was because the updater was not appropriately registered as a reader. But after ensuring that is the case, it still happens in 10% of startups. 
Lastly, we wonder why it would even work correctly *at all* when we failed to register the updater as a reader thread.
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