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

Ingo Molnar
2006-05-10T10:09:08Z

* 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