Messages in this thread | | | Subject | Re: [PATCH RFC tip/core/rcu 4/6] rcu: Clarify help text for RCU_BOOST_PRIO | From | Peter Zijlstra <> | Date | Thu, 26 Apr 2012 14:46:31 +0200 |
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On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote: > + This option specifies the real-time priority to which long-term > + preempted RCU readers are to be boosted. If you are working > + with a real-time application that has one or more CPU-bound > + threads running at a real-time priority level,
Then your application is broken ;-) the kernel is known to mis-behave under these circumstances since it doesn't get to run house-keeping tasks. RCU is just one of these and elevating it doesn't make it work.
> you should set > + RCU_BOOST_PRIO to a priority higher then the highest-priority > + real-time CPU-bound thread. The default RCU_BOOST_PRIO value > + of 1 is appropriate in the common case, which is real-time > + applications that do not have any CPU-bound threads.
Alternatively, 1 is the worst possible choice forcing people to consider the issue.
> + Some real-time applications might not have a single real-time > + thread that saturates a given CPU, but instead might have > + multiple real-time threads that, taken together, fully utilize > + that CPU. In this case, you should set RCU_BOOST_PRIO to > + a priority higher than the lowest-priority thread that is > + conspiring to prevent the CPU from running any non-real-time > + tasks. For example, if one thread at priority 10 and another > + thread at priority 5 are between themselves fully consuming > + the CPU time on a given CPU, then RCU_BOOST_PRIO should be > + set to priority 6 or higher.
I'd call this misleading, who's to say that preempting the 5 would yield enough time to complete the RCU work?
This all gets us back to the fun question of RCU delayed bandwidth budgeting.. ideally every 'task' that does call_rcu() should donate some of its budget towards the thread running the callback.
Anyway, I'd argue both the old and new description are bonkers.
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