Messages in this thread | | | Date | Sun, 22 Apr 2012 16:28:56 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH 12/16] rtmutex: turn the plist into an rb-tree. |
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On 04/11/2012 11:11 PM, Steven Rostedt wrote: > On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote: >> From: Peter Zijlstra<peterz@infradead.org> >> >> Turn the pi-chains from plist to rb-tree, in the rt_mutex code, >> and provide a proper comparison function for -deadline and >> -priority tasks. > > I have to ask. Why not just add a rbtree with a plist? That is, add all > deadline tasks to the rbtree and all others to the plist. As plist has a > O(1) operation, and rbtree does not. We are making all RT tasks suffer > the overhead of the rbtree. >
I basically got this patch from the v3 patchset and, since it applied perfectly and came from Peter, I assumed it was the right way to go ;-).
> As deadline tasks always win, the two may stay agnostic from each other. > Check first the rbtree, if it is empty, then check the plist. > > This will become more predominant with the -rt tree as it converts most > the locks in the kernel to pi mutexes. >
I see your point, but I'm not yet convinced that in the end the plist + rbtree implementation would win. AFAIK, the only O(1) plist operation is removal, beeing addition O(K) [K RT priorities]. Now, we have O(logn) [n elements] operations for rbtrees and we speed-up search with the leftmost pointer. So, are we sure that add complexity (and related checks) is needed here? I'm not against your point, I'm only asking :-).
Thanks a lot,
- Juri
>> >> This is done mainly because: >> - classical prio field of the plist is just an int, which might >> not be enough for representing a deadline; >> - manipulating such a list would become O(nr_deadline_tasks), >> which might be to much, as the number of -deadline task increases. >> >> Therefore, an rb-tree is used, and tasks are queued in it according >> to the following logic: >> - among two -priority (i.e., SCHED_BATCH/OTHER/RR/FIFO) tasks, the >> one with the higher (lower, actually!) prio wins; >> - among a -priority and a -deadline task, the latter always wins; >> - among two -deadline tasks, the one with the earliest deadline >> wins. >> >> Queueing and dequeueing functions are changed accordingly, for both >> the list of a task's pi-waiters and the list of tasks blocked on >> a pi-lock. >> >> Signed-off-by: Peter Zijlstra<peterz@infradead.org> >> Signed-off-by: Dario Faggioli<raistlin@linux.it> >> Signed-off-by: Juri Lelli<juri.lelli@gmail.com> > >
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