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    SubjectRe: [RFC 0/3] Slab Defrag / Slab Targeted Reclaim and general Slab API changes
    Christoph Lameter a écrit :
    > On Sat, 5 May 2007, Eric Dumazet wrote:
    >
    >>> C. Introduces a slab_ops structure that allows a slab user to provide
    >>> operations on slabs.
    >> Could you please make it const ?
    >
    > Sure. Done.

    thanks :)

    >
    >>> All of this is really not necessary since the compiler knows how to align
    >>> structures and we should use this information instead of having the user
    >>> specify an alignment. I would like to get rid of SLAB_HWCACHE_ALIGN
    >>> and kmem_cache_create. Instead one would use the following macros (that
    >>> then result in a call to __kmem_cache_create).
    >> Hum, the problem is the compiler sometimes doesnt know the target processor
    >> alignment.
    >>
    >> Adding ____cacheline_aligned to 'struct ...' definitions might be overkill if
    >> you compile a generic kernel and happens to boot a Pentium III with it.
    >
    > Then add ___cacheline_aligned_in_smp or specify the alignment in the
    > various other ways that exist. Practice is that most slabs specify
    > SLAB_HWCACHE_ALIGN. So most slabs are cache aligned today.

    Yes but this alignement is dynamic, not at compile time.

    include/asm-i386/processor.h:739:#define cache_line_size()
    (boot_cpu_data.x86_cache_alignment)

    So adding ____cacheline_aligned to 'struct file' for example would be a
    regression for people with PII or PIII

    >
    >> G. Being able to track the number of pages in a kmem_cache
    >>
    >>
    >> If you look at fs/buffer.c, you'll notice the bh_accounting, recalc_bh_state()
    >> that might be overkill for large SMP configurations, when the real concern is
    >> to be able to limit the bh's not to exceed 10% of LOWMEM.
    >>
    >> Adding a callback in slab_ops to track total number of pages in use by a given
    >> kmem_cache would be good.
    >
    > Such functionality exists internal to SLUB and in the reporting tool.
    > I can export that function if you need it.
    >
    >> Same thing for fs/file_table.c : nr_file logic
    >> (percpu_counter_dec()/percpu_counter_inc() for each file open/close) could be
    >> simplified if we could just count the pages in use by filp_cachep kmem_cache.
    >> The get_nr_files() thing is not worth the pain.
    >
    > Sure. What exactly do you want? The absolute number of pages of memory
    > that the slab is using?
    >
    > kmem_cache_pages_in_use(struct kmem_cache *) ?
    >
    > The call will not be too lightweight since we will have to loop over all
    > nodes and add the counters in each per node struct for allocates slabs.
    >
    >

    On a typical system, number of pages for 'filp' kmem_cache tends to be stable

    -bash-2.05b# grep filp /proc/slabinfo
    filp 234727 374100 256 15 1 : tunables 120 60 8 :
    slabdata 24940 24940 135
    -bash-2.05b# grep filp /proc/slabinfo
    filp 234776 374100 256 15 1 : tunables 120 60 8 :
    slabdata 24940 24940 168
    -bash-2.05b# grep filp /proc/slabinfo
    filp 234728 374100 256 15 1 : tunables 120 60 8 :
    slabdata 24940 24940 180
    -bash-2.05b# grep filp /proc/slabinfo
    filp 234724 374100 256 15 1 : tunables 120 60 8 :
    slabdata 24940 24940 174

    So revert nr_files logic to a single integer would be enough, even for NUMA

    int nr_pages_used_by_filp;
    int nr_pages_filp_limit;
    int filp_in_danger __read_mostly;

    static void callback_pages_in_use_by_filp(int inc)
    {
    int in_danger;

    nr_pages_used_by_filp += inc;

    in_danger = nr_pages_used_by_filp >= nr_pages_filp_limit;
    if (in_danger != filp_in_danger)
    filp_in_danger = in_danger;
    }

    struct file *get_empty_filp(void)
    {
    ...
    if (filp_in_danger && !capable(CAP_SYS_ADMIN))
    goto over;

    ...
    }


    void __init files_init(unsigned long mempages)
    {
    ...
    nr_pages_filp_limit = (mempages * 10) / 100; /* 10% for filp use */
    ...
    }
    -
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