Currently, LOG_BUF_SHIFT defaults to 17, which is 2 ^ 17 bytes = 128 KB,
and LOG_CPU_MAX_BUF_SHIFT defaults to 12, which is 2 ^ 12 bytes = 4 KB.
Half of 128 KB is 64 KB, so more than 16 CPUs are required for the value
to be used, as then the sum of contributions is greater than 64 KB for
the first time. My guess is, that the description was written with the
configuration values used in the SUSE in mind.
Cc: Luis R. Rodriguez <[email protected]>
Cc: [email protected]
Fixes: 23b2899f7f ("printk: allow increasing the ring buffer depending on the number of CPUs")
Signed-off-by: Paul Menzel <[email protected]>
---
init/Kconfig | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/init/Kconfig b/init/Kconfig
index d6a0b31b13dc..9dc607e3806f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -718,7 +718,7 @@ config LOG_CPU_MAX_BUF_SHIFT
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 64 CPUs are needed to trigger the allocation.
+ 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.
--
2.28.0
On Tue 2020-08-11 09:31:45, Paul Menzel wrote:
> Currently, LOG_BUF_SHIFT defaults to 17, which is 2 ^ 17 bytes = 128 KB,
> and LOG_CPU_MAX_BUF_SHIFT defaults to 12, which is 2 ^ 12 bytes = 4 KB.
>
> Half of 128 KB is 64 KB, so more than 16 CPUs are required for the value
> to be used, as then the sum of contributions is greater than 64 KB for
> the first time. My guess is, that the description was written with the
> configuration values used in the SUSE in mind.
>
> Cc: Luis R. Rodriguez <[email protected]>
> Cc: [email protected]
> Fixes: 23b2899f7f ("printk: allow increasing the ring buffer depending on the number of CPUs")
> Signed-off-by: Paul Menzel <[email protected]>
Great catch.
Reviewed-by: Petr Mladek <[email protected]>
Best Regards,
Petr