linux-kernel - Re: [PATCH 5/5] ipc/sem.c: alternatives to preempt

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Date:	Tue, 24 Jan 2012 15:51:45 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 5/5] ipc/sem.c: alternatives to preempt_disable()

On Sat, 2011-10-15 at 15:22 +0200, Manfred Spraul wrote:
> ipc/sem.c uses a custom wakeup scheme that relies on preempt_disable().
> On -RT, this causes increased latencies and debug warnings.
> 
> The patch adds two additional schemes:
> - one built around a completion - could be better for -RT kernels
> - one built around a spinlock - unfortunately it's broken
> - and the current one
> 
> Mike, Peter: Would the completion work on -rt?
> 
> My preferred solution would be the spinlock implementation:
> RT would use premptible spinlocks, mainline normal spinlocks.
> Thus both get the optimal implementation without any special code in
> ipc/sem.c.
> Unfortunately, I don't see how it could be fixed.

Sorry, I was convinced I replied to this, but I cannot actually find
anything in my send folder or elsewhere. Thanks for poking Andrew.

Yes I think it should work, and I'm afraid I have to agree with not
being able to make the spinlock thing work properly. Even if you were to
use arch_spin_* primitives you can still run into the 256 limit --
although not from the preempt_count in that case. Nor would arch_spin_
do what we need on -rt.


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