linux-kernel - Re: [PATCH -rt] ipc/sem: Rework semaphore wakeups

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Date:	Wed, 14 Sep 2011 21:23:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH -rt] ipc/sem: Rework semaphore wakeups

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@...llo.nl>
> > 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).
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