Re: [RFC][PATCH RT 0/2] futex priority based wakeup

[Ingo Molnar <mingo@elte.hu>]

* Sébastien Dugué <sebastien.dugue@bull.net> wrote:

>   in the current futex implementation, tasks are woken up in FIFO 
> order, (i.e. in the order they were put to sleep). For realtime 
> systems needing system wide strict realtime priority scheduling, tasks 
> should be woken up in priority order.
> 
>   This patchset achieves this by changing the futex hash bucket list 
> into a plist. Tasks waiting on a futex are enqueued in this plist 
> based on their priority so that they can be woken up in priority 
> order.

hm, i dont think this is enough. Basically, waking up in priority order 
is just the (easier) half of the story - what you want is to also 
propagate priorities when you block. We provided a complete solution via 
the PI-futex patchset (currently included in -mm).

In other words: as long as locking primitives go, i dont think real-time 
applications should use wakeup-priority-ordered futexes, they should use 
the real thing, PI futexes.

There is one exception: when a normal futex is used as a waitqueue 
without any contention properties. (for example a waitqueue for worker 
threads) But those are both rare, and typically dont muster tasks with 
different priorities - i.e. FIFO is good enough.

Also, there's a performance cost to this. Could you try to measure the 
impact to SCHED_OTHER tasks via some pthread locking benchmark?

	Ingo
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