Re: [arm64] kernel boot slowdown in v5.10.19 -> v5.10.42 update

From: Vijay Balakrishna
Date: Fri Jan 28 2022 - 13:03:12 EST




On 1/28/2022 4:06 AM, Catalin Marinas wrote:
On Mon, Jan 24, 2022 at 03:03:48PM -0800, Vijay Balakrishna wrote:
We noticed 150ms kernel boot slowdown back in June, 2021, when moving from
v5.10.19 to v5.10.42. This on a 8GB SoC. Only recently we investigated
this issue and found the regression is introduced by a change in map_mem()
(paging_init() -> map_mem() -> __map_memblock(), in particular "map all the
memory banks" for loop) by patch

2687275a5843d1089687f08fc64eb3f3b026a169
arm64: Force NO_BLOCK_MAPPINGS if crashkernel reservation is required

above is a follow up to

0a30c53573b07d5561457e41fb0ab046cd857da5
arm64: mm: Move reserve_crashkernel() into mem_init())

which deferred crashkernel reservation into mem_init().

The ~150ms slowdown disappears on booting without "crashkernel=.." on kernel
command-line.

Is CONFIG_RODATA_FULL_DEFAULT_ENABLED set in your config file? It may
cause the same slowdown. I suspect it's because we end up mapping the
RAM at the page granularity so more loops and slightly higher TLB
pressure. Not sure we can do much about >

Catalin,

Thanks for your reply.

CONFIG_RODATA_FULL_DEFAULT_ENABLED is not set in our config for boot time performance reasons.