CONFIG_LOG_CPU_MAX_BUF_SHIFT: CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)

General informations

The Linux kernel configuration item CONFIG_LOG_CPU_MAX_BUF_SHIFT:

• prompt: CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)
• type: int
• depends on: ( CONFIG_SMP ) && ( CONFIG_BASE_SMALL ) && ( CONFIG_PRINTK )
• defined in init/Kconfig
• found in Linux kernels: 3.17–3.19, 4.0–4.20, 5.0–5.19, 6.0–6.11, 6.12-rc+HEAD

Help text

This option allows to increase the default ring buffer size according to the number of CPUs. The value defines the contribution of each CPU as a power of 2. The used space is typically only few lines however it might be much more when problems are reported, e.g. backtraces.

The increased size means that a new buffer has to be allocated and the original static one is unused. It makes sense only on systems with more CPUs. Therefore this value is used only when the sum of contributions is greater than the half of the default kernel ring buffer as defined by LOG_BUF_SHIFT. The default values are set so that more than 16 CPUs are needed to trigger the allocation.

Also this option is ignored when "log_buf_len" kernel parameter is used as it forces an exact (power of two) size of the ring buffer.

The number of possible CPUs is used for this computation ignoring hotplugging making the computation optimal for the worst case scenario while allowing a simple algorithm to be used from bootup.

Examples shift values and their meaning: 17 => 128 KB for each CPU 16 => 64 KB for each CPU 15 => 32 KB for each CPU 14 => 16 KB for each CPU 13 => 8 KB for each CPU 12 => 4 KB for each CPU

Hardware

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Sources

This page is automaticly generated with free (libre, open) software lkddb(see lkddb-sources).

The data is retrived from:

• Linux kernel
• Linux Kernel Driver DataBase (LKDDb)

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