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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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