On Tue, 22 Mar 2005, Paul E. McKenney wrote: > On Tue, Mar 22, 2005 at 09:55:26AM +0100, Esben Nielsen wrote: > > On Mon, 21 Mar 2005, Paul E. McKenney wrote: > [ . . . ] > > > On Mon, Mar 21, 2005 at 12:23:22AM +0100, Esben Nielsen wrote: > > > This is in some ways similar to the K42 approach to RCU (which they call > > > "generations"). Dipankar put together a similar patch for Linux, but > > > the problem was that grace periods could be deferred for an extremely > > > long time. Which I suspect is what you were calling out as causing > > > RCU batches never to run. > > > > That is where the preempt_by_nonrt_disable/enable() is supposed to help: > > Then it can't take longer than the normal kernel in the situation where > > there is no RT tasks running. RT tasks will prolong the grace periods if > > they go into RCU regions, but they are supposed to be relatively small - > > and deterministic! > > The part that I am missing is how this helps in the case where a non-RT > task gets preempted in the middle of an RCU read-side critical section > indefinitely. Or are you boosting the priority of any task that > enters an RCU read-side critical section?
Yes in effect: I set the priority to MAX_RT_PRIO. But actually I am playing around (when I get time for it that is :-( ) with cheaper solution: I assume you enter these regions where you don't want to be preempted by non-RT tasks are relatively short. Therefore the risc of getting preempted is small. Moving the priority is expensive since you need to lock the runqueue. I only want to do the movement when there is an preemption. Therefore I added code in schedule() to take care of it: If a task is in a rcu-read section, is non-RT and is preempted it's priority is set to MAX_RT_PRIO for the time being. It will keep that priority until the priority is recalculated, but that shouldn't hurt anyone. I am not happy about adding code to schedule() but setting the priority in there is very cheap because it already has the lock on the runqueue. Furthermore, I assume it only happens very rarely. In the execution of schedule() my code only takes a single test on wether the previous task was in a rcu-section or not. That is not very much code. I have not yet tested it (no time :-( ) > [...] > > > Yes, but this is true of every other lock in the system as well, not? > > > > Other locks are not globaly used but only used for a specific subsystem. > > On a real-time system you are supposed to know which subsystems you can > > call into and still have a low enough latency as each subsystem has it's > > own bound. But with a global RCU locking mechanism all RCU using code is > > to be regarded as _one_ such subsystem. > > Yep. As would the things protected by the dcache lock, task list lock, > and so on, right? Yep > > Thanx, Paul > Esben - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
