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Subject[PATCH] x86: OLPC: speed up device tree creation during boot

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 hack, 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.)

Signed-off-by: Andres Salomon <dilinger@queued.net>
---
arch/x86/kernel/olpc_dt.c | 27 +++++++++++++++++++++++----
1 files changed, 23 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/olpc_dt.c b/arch/x86/kernel/olpc_dt.c
index f660a11..44dd2ae 100644
--- a/arch/x86/kernel/olpc_dt.c
+++ b/arch/x86/kernel/olpc_dt.c
@@ -123,16 +123,35 @@ static int __init olpc_dt_pkg2path(phandle node, char *buf,
}

static unsigned int prom_early_allocated __initdata;
+#define DT_CHUNK_SIZE (1<<12)

void * __init prom_early_alloc(unsigned long size)
{
+ static u8 *mem = NULL;
+ static size_t free_mem = 0;
void *res;

- res = alloc_bootmem(size);
- if (res)
- memset(res, 0, size);
+ if (free_mem >= size) {
+ /* allocate from the local cache */
+ free_mem -= size;
+ res = mem;
+ mem += size;
+ return res;
+ }

- prom_early_allocated += size;
+ /*
+ * To mimimize the number of allocations, grab 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(DT_CHUNK_SIZE);
+ if (res) {
+ prom_early_allocated += DT_CHUNK_SIZE;
+ memset(res, 0, DT_CHUNK_SIZE);
+ free_mem = DT_CHUNK_SIZE - size;
+ mem = res + size;
+ }

return res;
}
--
1.5.6.5


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