On Thu, Jul 16, 2009 at 09:17:32AM -0600, Chris Friesen wrote: > If a high-priority task A makes a syscall that requires a lock currently > held by a sleeping low-priority task C, and there is a medium priority B > task that wants to run, the classic scenario for priority inversion has > been achieved. I think you don't really mean "sleeping" low-priority task C, since then the priority inheritance would do no good. I guess you mean that C has been/is preempted by B (and for global SMP, there is some other medicum priority task B' that is eligible to run on A's processor). That could be a priority inversion scenario. BTW, if migration is allowed the probability of this kind of thing (and hence the payoff for PIP) goes down rapidly with the number of processors. > I know of at least one example with millions of lines of code being > ported to linux from another OS. The scheduling requirements are fairly > lax but deadlock due to priority inversion is a highly likely. They > compare PI and PP, see that PP requires up-front analysis, so they > enable PI. > > I suspect there are other similar cases where deadlock is the real > issue, and hard realtime isn't a concern (but low latency may be > desirable). PI is simple to enable and doesn't require any thought on > the part of the app writer. I'm confused by your reference to deadlock. Priority inheritance does not prevent deadlock, even on a single processor. > At least for POSIX, both PI and PP mutexes can suspend while the lock is > held. From the user's point of view, the only difference between the > two is that PP bumps the lock holder's priority always, while PI bumps > the priority only if/when necessary. You are right that POSIX missed the point of priority ceilings, by allowing suspension. However, there is still a difference in context-switching overhead. Worst-case, you have twice as many context switches per critical section with PIP as with PP. In any case, for a multiprocessor, PP is not enough. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
