On Sat, 2008-05-31 at 20:25 -0700, Christoph Lameter wrote: > On Fri, 30 May 2008, Peter Zijlstra wrote: > > > Remember, horribly long on -rt is in the order of ~30us. > > > > See for instance the path: > > > > flush_slab() > > slab_lock() <-- bit_spinlock > > deactive_slab() > > loop over objects > > looping over all objects is rarely ever done. > > > unfreeze_slab() > > add_partial() > > spin_lock(&n->list_lock) <-- another lock > > > > all of that runs with preempt disabled, and worse it can spin for some > > indefinite amount of time. > > > > That is totally unacceptable on -rt, so we need to make it all > > preemptible and use sleeping locks (IRQ inversion can't happen, for the > > only thing that runs in hardirq is try_to_wake_up()). > > The way I thought about -rt was that it would convert the > spinlocks to semaphores. Isnt it possible to sleep on a bit spinlock in > -rt? Aside from that I would have thought that the simple disabling of > rescheduling on another processor in some context would gets us there. > > Never thought about the details though. s/semaphores/PI-mutexes/ yes you can do a bit-sleep-lock ala lock_page. But for any sleeping lock we must not have preemption disabled, coming right back to the initial problem. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
