linux-kernel - Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

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Message-ID: <20071108002027.GV17536@waste.org>
Date:	Wed, 7 Nov 2007 18:20:27 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Marin Mitov <mitov@...p.bas.bg>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>
Subject: Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

On Wed, Nov 07, 2007 at 12:30:45PM -0800, Andrew Morton wrote:
> Ow.  Yes, from my reading delay_tsc() can return early (or after
> heat-death-of-the-universe) if the TSCs are offset and if preemption
> migrates the calling task between CPUs.
> 
> I suppose a lameo fix would be to disable preemption in delay_tsc().

preempt_disable is lousy documentation here. This and other cases
(lots of per_cpu users, IIRC) actually want a migrate_disable() which
is a proper subset. We can simply implement migrate_disable() as
preempt_disable() for now and come back later and implement a proper
migrate_disable() that still allows preemption (and thus avoids the
latency).

-- 
Mathematics is the supreme nostalgia of our time.
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