Messages in this thread | | | Date | Sat, 8 Jul 2006 14:59:37 +0100 (BST) | Subject | Re: [PATCH -rt] catch put_task_struct RCU handling up to mainline | From | Esben Nielsen <> |
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On Fri, 7 Jul 2006, Paul E. McKenney wrote:
> On Fri, Jul 07, 2006 at 11:56:00PM +0100, Esben Nielsen wrote: >> On Fri, 7 Jul 2006, Paul E. McKenney wrote: >> >>> Hello! >>> >>> Due to the separate -rt and mainline evolution of RCU signal handling, >>> the -rt patchset now makes each task struct go through two RCU grace >>> periods, with one call_rcu() in release_task() and with another >>> in put_task_struct(). Only the call_rcu() in release_task() is >>> required, since this is the one that is associated with tearing down >>> the task structure. >>> >>> This patch removes the extra call_rcu() in put_task_struct(), synching >>> this up with mainline. Tested lightly on i386. >>> >> >> The extra call_rcu() has an advantage: >> It defers work away from the task doing the last put_task_struct(). >> It could be a priority 99 task with hard latency requirements doing >> some PI boosting, forinstance. The extra call_rcu() defers non-RT work to >> a low priority task. This is in generally a very good idea in a real-time >> system. >> So unless you can argue that the work defered is as small as the work of >> doing a call_rcu() I would prefer the extra call_rcu(). > > I would instead argue that the only way that the last put_task_struct() > is an unrelated high-priority task is if it manipulating an already-exited > task. In particular, I believe that the sys_exit() path prohibits your > example of priority-boosting an already-exited task by removing the > exiting task from the various lists before doing the release_task() > on itself. > > Please let me know what I am missing here!
You could very well be right (I don't know the details that well). But in that case the get/put_task_struct() in the PI code is not needed? I think, however, it is needed because the task doing the (de)boosting gets a pointer to a task, enables preemption and drops all locks. It then uses the pointer. The task could have been deleted a long time ago if it wasn't used protected by get/put_task_struct().
This is an examble of why using reference counting in a RT system is a bad idea: Suddenly a highpriority task can end up doing the cleanup for low priority tasks.
The work should be defered to a low priority task. Using rcu is probably overkill because it also introduces other delays. A tasklet or a dedicated task would be better.
Esben
> > Thanx, Paul > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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