https://rajanvaja.wordpress.com/2015/06/04/how-to-change-linux-kernel-dmesg-buffer-size/
Printk is implemented by using a ring buffer in the kernel with a size of __LOG_BUF_LEN bytes where __LOG_BUF_LEN equals (1 << CONFIG_LOG_BUF_SHIFT).
dmesg buffer size can be changed using “CONFIG_LOG_BUF_SHIFT” Kconfig.
As per kernel source (4.1.0), default value of this config is 17 (i.e. 1 << 17 = 128KB). The range for this config is 12 to 21.
Addign below line in defconfig sets
Note: If invalid value (out of range value) is set, default value is taken.
CONFIG_LOG_BUF_SHIFT can be changed from menuconfig or from defconfig as below:
CONFIG_LOG_BUF_SHIFT=21
https://elixir.bootlin.com/linux/v4.9.253/source/init/Kconfig#L823
config LOG_BUF_SHIFT
int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
range 12 25 if !H8300
range 12 19 if H8300
default 17
depends on PRINTK
help
Select the minimal kernel log buffer size as a power of 2.
The final size is affected by LOG_CPU_MAX_BUF_SHIFT config
parameter, see below. Any higher size also might be forced
by "log_buf_len" boot parameter.
Examples:
17 => 128 KB
16 => 64 KB
15 => 32 KB
14 => 16 KB
13 => 8 KB
12 => 4 KB