On Mon, 2007-08-06 at 15:29 +0200, Peter Zijlstra wrote: > On Mon, 2007-08-06 at 17:18 +0400, Oleg Nesterov wrote: > > > Yes, I still disagree with the whole idea because I hope we can make > > something more simpler to solve the problem, but I must admit I don't > > quite understand what the problem is. > > > > So, please consider a noise from my side as my attempt to help. And > > in fact, I am very curious about -rt tree, just I never had a time > > to study it :) > > > Well, the thing is, suppose we have 2 drivers both using keventd say a > NIC and some USB thingy. > > Now the NIC is deemed important hand gets irq thread prio 90, and the > USB could not be cared less about and gets 10 (note that on -rt irq > handlers are threaded and run SCHED_FIFO). > > So now you can get priority inversion in keventd. Say the USB thingy > schedules a work item which will be executed. Then during the execution > of this work the NIC will also schedule a work item. Now the NIC (fifo > 90) will have to wait for the USB work (fifo 10) to complete. /me hits himself. of course today everything will run on whatever prio keventd ends up, regardless of the prio of the submitter. still this does not change the fundamental issue of a high prio piece of work waiting on a lower prio task. > The typical solution is priority inheritance, where the highest prio of > any waiter is propagated to the currently running work, so that it can > finish and get on with the more important work. > > > So these patches aimed to provide proper PI in the workqueue structure > to avoid this problem. > > However as you rightly noted, this horribly breaks the barrier/flush > semantics. > > I suspect most of the barrier/flush semantics could be replaced with > completions from specific work items. Doing this will be a lot of work > though. > > I hope this rambling is not confusing you any further :-) - 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
