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

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Message-ID: <20071108154353.GT19691@waste.org>
Date:	Thu, 8 Nov 2007 09:43:53 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Thu, Nov 08, 2007 at 10:11:21AM +0100, Peter Zijlstra wrote:
> 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

Hmm, I'm surprised it's more than a one-liner. Something like

if (cannot_migrate(task))
	continue;

But I'm certainly not the expert here. Perhaps this one-liner introduces the
"unplannable O(N) overhead" Ingo mentions in the email you referenced.

> 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.

Ok, so maybe we've got implementation issues to tackle. But still: a)
preempt_disable is inherently misdocumentation (preemption is not the
real problem) and b) preemption is a bigger hammer than needed. At the
very least, we should introduce migrate_disable as an alias for
preempt_disable so we can document intent.

At any rate, in the current context, we're talking about actually
disabling preempt in a *delay loop*. Let's find a fix for that.

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

Heh, I'd completely forgotten about this thread and thought I was
having an original idea! Must have never seen Ingo's followup.

Ingo's complaint about it being "a per-task BKL" is interesting. I've
occassionally wondered if it makes sense to make some of these
primitives take a "documentation parameter" - a pointer to an object
that does nothing more than indicate the object of interest.

-- 
Mathematics is the supreme nostalgia of our time.
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