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

[Catalin Marinas]

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