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
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