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

[Vijay Balakrishna]

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.

Please let us know if it known/understood or addressed already or being 
looked into.  Any insight is appreciated.

Thanks,
Vijay

console output collected with debug prints --

[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd083]
[    0.000000] Linux version 
5.10.42.6-xxx-standard-2008.3.0-local-snapshot-01210432 
(oe-user@oe-host) (clang version 10.[    0.000001] sched_clock: 64 bits 
at 125MHz, resolution 8ns, wraps every 4398046511100ns
[    0.000003]          2.560781952 MSFT: kernel boot start
[    0.000484] Machine model: xxx
[    0.009379] earlycon: uart8250 at MMIO32 0x0000000068a10000 (options '')
[    0.009383] printk: bootconsole [uart8250] enabled
[    0.009828] before paging_init()
[    0.009977] before map_mem()
[    0.009978] after memblock_mark_nomap()
[    0.152388] after __map_memblock() for loop
[    0.152391] after __map_memblock() kernel_start, kernel_end
[    0.152393] after memblock_clear_nomap
[    0.152394] after map_mem()
[    0.152401] after paging_init()
[    0.153273] NUMA: No NUMA configuration found
[    0.153278] NUMA: Faking a node at [mem 
0x0000000080000000-0x000000097fffffff]
[    0.153282] NUMA: NODE_DATA [mem 0x97efec5c0-0x97efedfff]
[    0.153316] Zone ranges:
[    0.153319]   Normal   [mem 0x0000000080000000-0x000000097fffffff]
[    0.153323]   Device   empty
[    0.153325] Movable zone start for each node
[    0.153326] Early memory node ranges
[    0.153328]   node   0: [mem 0x0000000080000000-0x000000008adfffff]
[    0.153330]   node   0: [mem 0x000000008ae00000-0x000000008f223fff]
[    0.153331]   node   0: [mem 0x000000008f224000-0x00000000ffffffff]
[    0.153333]   node   0: [mem 0x0000000880000000-0x00000008800fffff]
[    0.153334]   node   0: [mem 0x0000000880100000-0x000000097fffffff]
[    0.153337] Initmem setup node 0 [mem 
0x0000000080000000-0x000000097fffffff]
[    0.153340] On node 0 totalpages: 1572864
[    0.153342]   Normal zone: 24576 pages used for memmap
[    0.153343]   Normal zone: 0 pages reserved
[    0.153345]   Normal zone: 1572864 pages, LIFO batch:63
[    0.154507] crashkernel reserved: 0x0000000968e00000 - 
0x0000000978e00000 (256 MB)

Console output from PXE boot without "crashkernel=256M" boot parameter --
[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd083]
[    0.000000] Linux version 
5.10.42.6-xxx-standard-2008.3.0-local-snapshot-01210432 
(oe-user@oe-host) (clang version 10.[    0.000001] sched_clock: 64 bits 
at 125MHz, resolution 8ns, wraps every 4398046511100ns
[    0.000003]         14.765446920 MSFT: kernel boot start
[    0.000484] Machine model: xxx
[    0.009400] earlycon: uart8250 at MMIO32 0x0000000068a10000 (options '')
[    0.009404] printk: bootconsole [uart8250] enabled
[    0.009835] before paging_init()
[    0.009986] before map_mem()
[    0.009987] after memblock_mark_nomap()
[    0.010154] after __map_memblock() for loop
[    0.010156] after __map_memblock() kernel_start, kernel_end
[    0.010158] after memblock_clear_nomap
[    0.010159] after map_mem()
[    0.010163] after paging_init()
[    0.011002] NUMA: No NUMA configuration found
[    0.011005] NUMA: Faking a node at [mem 
0x0000000080000000-0x000000097fffffff]
[    0.011009] NUMA: NODE_DATA [mem 0x97fbc65c0-0x97fbc7fff]
[    0.011037] Zone ranges:
[    0.011038]   Normal   [mem 0x0000000080000000-0x000000097fffffff]
[    0.011041]   Device   empty
[    0.011043] Movable zone start for each node
[    0.011044] Early memory node ranges
[    0.011046]   node   0: [mem 0x0000000080000000-0x000000008adfffff]
[    0.011048]   node   0: [mem 0x000000008ae00000-0x000000008f223fff]
[    0.011049]   node   0: [mem 0x000000008f224000-0x00000000ffffffff]
[    0.011050]   node   0: [mem 0x0000000880000000-0x00000008800fffff]
[    0.011052]   node   0: [mem 0x0000000880100000-0x000000097fffffff]
[    0.011054] Initmem setup node 0 [mem 
0x0000000080000000-0x000000097fffffff]
[    0.011056] On node 0 totalpages: 1572864
[    0.011058]   Normal zone: 24576 pages used for memmap
[    0.011060]   Normal zone: 0 pages reserved
[    0.011061]   Normal zone: 1572864 pages, LIFO batch:63