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    SubjectRe: Per-CPU data as a structure
    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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