Subject: Re: is minimum udelay() not respected in preemptible SMP
	kernel-2.6.23?
From: Peter Zijlstra <peterz@infradead.org>
To: Matt Mackall <mpm@selenic.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, 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>
In-Reply-To: <20071108002027.GV17536@waste.org>
References: <200711071921.52330.mitov@issp.bas.bg>
	 <20071107123045.c6d4b855.akpm@linux-foundation.org>
	 <20071108002027.GV17536@waste.org>
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Date: Thu, 08 Nov 2007 10:11:21 +0100
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On Wed, 2007-11-07 at 18:20 -0600, Matt Mackall wrote:

>  This and other cases
> (lots of per_cpu users, IIRC) actually want a migrate_disable() which
> is a proper subset. 

The disadvantage of migrate_disable() is that it complicates the
load-balancer but more importantly, that it does bring a form of
latencies with it that are hard to measure. Using preempt_disable() for
these current per-cpu users basically forces them to keep it short.

   Which is a GOOD (tm) thing.

If we go overboard with this migrate_disable() stuff we can end up with
a very hard to analyse system that sporadically does weird stuff.

So, please, don't start that again.

Also see:
http://lkml.org/lkml/2007/7/23/338

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