linux-kernel - Re: lockdep complaints in slab allocator

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Message-ID: <alpine.DEB.2.00.0912011436420.27500@chino.kir.corp.google.com>
Date:	Tue, 1 Dec 2009 14:41:44 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Matt Mackall <mpm@...enic.com>
cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Nick Piggin <npiggin@...e.de>
Subject: Re: lockdep complaints in slab allocator

On Mon, 30 Nov 2009, Matt Mackall wrote:

> And it's not even something that -most- of embedded devices will want to
> use, so it can't be keyed off CONFIG_EMBEDDED anyway. If you've got even
> 16MB of memory, you probably want to use a SLAB-like allocator (ie not
> SLOB). But there are -millions- of devices being shipped that don't have
> that much memory, a situation that's likely to continue until you can
> fit a larger Linux system entirely in a <$1 microcontroller-sized device
> (probably 5 years off still).
> 

What qualifying criteria can we use to automatically select slob for a 
kernel or the disqualifying criteria to automatically select slub as a 
default, then?  It currently depends on CONFIG_EMBEDDED, but it still 
requires the user to specifically chose the allocator over another.  Could 
we base this decision on another config option enabled for systems with 
less than 16MB?

> This thread is annoying. The problem that triggered this thread is not
> in SLOB/SLUB/SLQB, nor even in our bog-standard 10yo deep-maintenance
> known-to-work SLAB code. The problem was a FALSE POSITIVE from lockdep
> on code that PREDATES lockdep itself. There is nothing in this thread to
> indicate that there is a serious problem maintaining multiple
> allocators. In fact, considerably more time has been spent (as usual)
> debating non-existent problems than fixing real ones.
> 

We could move the discussion on the long-term maintainable aspects of 
multiple slab allocators to a new thread if you'd like.
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