On Wed, Mar 23, 2005 at 12:44:52PM +0100, Esben Nielsen wrote: > 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.
Interesting approach -- could come in handy. > I have not yet tested it (no time :-( ) Well, being as I haven't got the lock-based scheme fully running yet, I can't give you too much trouble about that. :-/ Thanx, Paul > > [...] > > > > 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/