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

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Date:	Wed, 14 Sep 2011 20:48:29 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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.

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]

--
     Manfred

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