On Wed, 2011-09-14 at 20:48 +0200, Manfred Spraul wrote: > On 09/14/2011 11:57 AM, Peter Zijlstra wrote: > > Subject: ipc/sem: Rework semaphore wakeups > > From: Peter Zijlstra<a.p.zijlstra@xxxxxxxxx> > > Date: Tue Sep 13 15:09:40 CEST 2011 > > > > Current sysv sems have a weird ass wakeup scheme that involves keeping > > preemption disabled over a potential O(n^2) loop and busy waiting on > > that on other CPUs. > Have you checked that the patch improves the latency? > Note that the busy wait only happens if there is a simultaneous timeout > of a semtimedop() and a true wakeup. > > The code does: > > spin_lock() > preempt_disable(); > usually_very_simple_but_worstcase_O_2 > spin_unlock() > usually_very_simple_but_worstcase_O_1 > preempt_enable(); > > with your change, it becomes: > > spin_lock() > usually_very_simple_but_worstcase_O_2 > usually_very_simple_but_worstcase_O_1 > spin_unlock() > > The complex ops remain unchanged, they are still under a lock. preemptible lock (aka pi-mutex) on -rt, so no weird latencies. > What about removing the preempt_disable? > It's only there to cover a rare race on uniprocessor preempt systems. > (a task is woken up simultaneously due to timeout of semtimedop() and a > true wakeup) > > Then fix the that race - something like the attached patch [obviously > buggy - see the fixme] sched_yield() is always a bug, as is it here. Its an life-lock if the woken task is of higher priority than the waking task. A higher prio FIFO task calling sched_yield() in a loop is just that, a loop, starving the lower prio waker. If you've got enough medium prio tasks around to occupy all other cpus, you're got indefinite priority inversion, so even on smp its a problem. But yeah its not the prettiest of solutions but it works.. see that other patch with the wake-list stuff for something that ought to work for both rt and mainline (except of course it doesn't actually work). -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
