Date: Fri, 4 May 2007 10:55:26 +0200
From: Eric Dumazet <dada1@cosmosbay.com>
To: "Julio M. Merino Vidal" <jmerino@ac.upc.edu>
Cc: Andi Kleen <andi@firstfloor.org>, linux-kernel@vger.kernel.org
Subject: Re: Per-CPU data as a structure
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On Fri, 04 May 2007 10:36:37 +0200
"Julio M. Merino Vidal" <jmerino@ac.upc.edu> wrote:


> 
> Anyway, what do you think about adding the above text to the code (percpu.h
> maybe) as documentation?  See the patch below.  (Dunno if the Signed-off-by
> line is appropriate as most of the text is yours.)
> 
> Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>
> 
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index 600e3d3..b8e8b8c 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -1,6 +1,21 @@
>  #ifndef __LINUX_PERCPU_H
>  #define __LINUX_PERCPU_H
>  
> +/*
> + * percpu provides a mechanism to define variables that are specific to 
> each
> + * CPU in the system.
> + *
> + * Each variable is defined as an independent array of NR_CPUS elements.
> + * This approach is used instead of a per-CPU structure because it has the
> + * following advantages:
> + * - Independent maintenance: a source file can define new per-CPU
> + *   variables without distorting others.
> + * - Fast access and relatively compact code.
> + * - Avoids false sharing by keeping cache lines of different CPUs 
> separate.
> + * - Doesn't waste a lot of memory in padding like NR_CPUs arrays usually
> + *   need to to avoid the previous point.
> + */
> +
>  #include <linux/spinlock.h> /* For preempt_disable() */
>  #include <linux/slab.h> /* For kmalloc() */
>  #include <linux/smp.h>

Documentation is good, and percpu probably misses one, but please add it in a Documentation/percpu.txt file, because it's the right place.

You then can really have an extensive documentation, and you wont slow down kernel compiles...

I suggest you document all variants (get_cpu_var(), __get_cpu_var(), ...) with examples of use

Also, please note that per cpu data is not allocated * NR_CPUS, but depends on possible cpus. So if you boot an SMP kernel on a one CPU desktop, kernel allocates only the needed space.

So per_cpu data has also a space saving argument against structures declared with [NR_CPUS] arrays.

Thank you

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