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I hope to check the value of log_buf_len via sysctl, but sysctl -a doesn't show it, why and is there any other way to check its value

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The value of log_buf_len is set once and for all when the kernel is configured and built OR when it is booted with a parameter log_buf_len=NNN: you cannot change it after that. sysctl generally concerns itself with things that you can change in the running kernel: that's probably the reason why log_buf_len is not included.

You can find the configured value of LOG_BUF_SHIFT by just grepping through the kernel config file (assuming that it's available). For example, on my Fedora 29 system, I do

$ grep CONFIG_LOG_BUF_SHIFT /boot/config-5.2.11-100.fc29.x86_64 
CONFIG_LOG_BUF_SHIFT=18

log_buf_len is equal to 2^18 then UNLESS it has been set through the boot command line.

If that is the case (or if you don't have access to the config file), then the only way I know is to use gdb to examine the running kernel, but I haven't done that in many years and I really can't remember all the steps - the main difficulty is to get debugging symbols for the kernel you are running. Somebody else will have to provide those instructions though (or perhaps provide a simpler method; e.g. it is possible to write a kernel module that prints out the value when it is loaded - there are many tutorials on writing simple kernel modules, e.g. here: that tutorial dates from the days of the 2.6 kernel and things may be different today but I (naively?) don't expect things to be very different).

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