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

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Date:	Thu, 08 Nov 2007 10:11:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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, 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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