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SubjectRe: [patch] Real-Time Preemption, plist fixes

* Esben Nielsen <simlo@phys.au.dk> wrote:

> Sorted lists works deterministicly O(1) on UP if no owner of the lock
> blocks while having the lock. On SMP or worse if an owner blocks in
> the lock, the wait list can grow very long. Thus insertion of new
> elements takes a long time - with preemption disabled :-(

the wait list can grow only as long as the max # of RT tasks is. Sorted
lists become 'O(1)' if we added some code that globally limits the
number of RT tasks to say 50. E.g. /proc/sys/kernel/max_nr_RT_tasks. A
user can override it if he needs more RT tasks. There can be an
arbitrary number of SCHED_OTHER tasks.

(note that on Linux there is a RAM-dependent 'max # of tasks' ulimit
which is never 'infinity', so theoretically the sorted lists are "O(1)"
too. But this is nitpicking.)
> If this is supposed to be used for user-space PI as well I would say
> it would have to be completely bounded, i.e. plists are certainly
> needed. [...]

yes, it's supposed to be used for user-space PI too. What do you mean by
'completely bounded'. Do you consider the current worst-case O(100)
property of plists a 'completely bounded' solution?

i dont think fusyn's should be made available to non-RT tasks. If this
restriction is preserved then fusyn's would become O(max_nr_RT_tasks)
too.

Ingo
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