Re: [PATCH 12/16] rtmutex: turn the plist into an rb-tree.

[Juri Lelli]

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@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > Signed-off-by: Dario Faggioli<raistlin@xxxxxxxx>
> > Signed-off-by: Juri Lelli<juri.lelli@xxxxxxxxx>
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