Date: Wed, 7 Nov 2007 18:20:27 -0600
From: Matt Mackall <mpm@selenic.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Marin Mitov <mitov@issp.bas.bg>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
       Andi Kleen <ak@suse.de>
Subject: Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?
Message-ID: <20071108002027.GV17536@waste.org>
References: <200711071921.52330.mitov@issp.bas.bg> <20071107123045.c6d4b855.akpm@linux-foundation.org>
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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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