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    Subject[PATCH 3/3] x86: OLPC: speed up device tree creation during boot (v3)

    Calling alloc_bootmem() for tiny chunks of memory over and over is really
    slow; on an XO-1, it caused the time between when the kernel started
    booting and when the display came alive (post-lxfb probe) to increase
    to 44s. This patch optimizes the prom_early_alloc function by
    calling alloc_bootmem for 4k-sized blocks of memory, and handing out
    chunks of that to callers. With this patch, the time between kernel load
    and display initialization decreased to 23s. If there's a better way to
    do this early in the boot process, please let me know.

    (Note: increasing the chunk size to 16k didn't noticably affect boot time,
    and wasted 9k.)

    v3: fix wasted memory buglet found by Milton Miller, and style fixes.
    v2: reorder prom_early_alloc as suggested by Grant Likely.

    Signed-off-by: Andres Salomon <dilinger@queued.net>
    ---
    arch/x86/platform/olpc/olpc_dt.c | 28 +++++++++++++++++++++++-----
    1 files changed, 23 insertions(+), 5 deletions(-)

    diff --git a/arch/x86/platform/olpc/olpc_dt.c b/arch/x86/platform/olpc/olpc_dt.c
    index 7054697..6614ec4 100644
    --- a/arch/x86/platform/olpc/olpc_dt.c
    +++ b/arch/x86/platform/olpc/olpc_dt.c
    @@ -126,14 +126,32 @@ static unsigned int prom_early_allocated __initdata;

    void * __init prom_early_alloc(unsigned long size)
    {
    + static u8 *mem;
    + static size_t free_mem;
    void *res;

    - res = alloc_bootmem(size);
    - if (res)
    - memset(res, 0, size);
    -
    - prom_early_allocated += size;
    + if (free_mem < size) {
    + const size_t chunk_size = max(PAGE_SIZE, size);
    +
    + /*
    + * To mimimize the number of allocations, grab at least 4k of
    + * memory (that's an arbitrary choice that matches PAGE_SIZE on
    + * the platforms we care about, and minimizes wasted bootmem)
    + * and hand off chunks of it to callers.
    + */
    + res = alloc_bootmem(chunk_size);
    + if (!res)
    + return NULL;
    + prom_early_allocated += chunk_size;
    + memset(res, 0, chunk_size);
    + free_mem = chunk_size;
    + mem = res;
    + }

    + /* allocate from the local cache */
    + free_mem -= size;
    + res = mem;
    + mem += size;
    return res;
    }

    --
    1.7.2.3




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