linux-kernel - Re: [patch 2/2] slub: add Documentation/ABI/testing/sysfs-kernel-slab

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Message-ID: <alpine.DEB.2.00.0904241626590.10046@chino.kir.corp.google.com>
Date:	Fri, 24 Apr 2009 16:35:01 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] slub: add
 Documentation/ABI/testing/sysfs-kernel-slab

On Thu, 23 Apr 2009, Christoph Lameter wrote:

> > I'm more concerned with the userspace API.  You're comfortable with
> > letting userspace determine what allocator the kernel is using either by
> > testing for the presence of certain slub or slqb-specific files or
> > checking the config?
> 
> Why would userspace have to test? If the fields have the same semantics
> then we are fine.
> 

For a robust init script to determine which allocator the kernel is using, 
it must either do `zgrep CONFIG_SLQB=y /proc/config.gz` or 
if [ -e /sys/kernel/slab/cache/hiwater ], for example.  The latter is more 
delicate because the API could change or (even worse) another allocator, 
present or future, could include a `hiwater' attribute.

> > That should be fine since there's parity among the files that both
> > allocators share, but seems fragile if an allocator adds a file that was
> > previously only applicable to the other or the API happens to change.
> 
> Right. The newly added file must not be in use by another allocator.
> 

Yeah, at least with different semantics or side effects.

> > My thinking was that /sys/kernel/slqb would be a permanent reference point
> > that userspace could easily test to determine what API is available.
> 
> The API is hopefully generic enought to accomodate all allocators.
> 

Ok, fair enough.

SLQB will need to update this file and hopefully it doesn't become too 
terribly complex.
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