[PATCH v2] printk: increase the ring buffer for more than 16 CPUs by default

[PATCH v2] printk: increase the ring buffer for more than 16 CPUs by default

[Sang-Heon Jeon]

The default LOG_BUF_SHIFT is 17, which means 128 KB, and
LOG_CPU_MAX_BUF_SHIFT is 12, which means 4 KB per CPU.

By default the ring buffer is increased when
(num_possible_cpus() - 1) * 4 KB > 64 KB, which requires more than 17
CPUs.

Remove the explicit subtraction of one, so the ring buffer is increased
for more than 16 CPUs, which is a power of two. Also fix the outdated
comment.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
Changes from v1 [1]
- Adjust default threshold to power of two, as suggested by Petr

[1] https://lore.kernel.org/all/20260921163828.111061-1-ekffu200098@gmail.com/
---
 kernel/printk/printk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 00f8e1291564..1ba440c1193e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1085,9 +1085,9 @@ static void __init log_buf_add_cpu(void)
 	if (num_possible_cpus() == 1)
 		return;
 
-	cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MAX_BUF_LEN;
+	cpu_extra = num_possible_cpus() * __LOG_CPU_MAX_BUF_LEN;
 
-	/* by default this will only continue through for large > 64 CPUs */
+	/* by default this will only continue through for more than 16 CPUs */
 	if (cpu_extra <= __LOG_BUF_LEN / 2)
 		return;
 

base-commit: b03d22ec8186f3f6b40b95a5a678100cf80116cb
-- 
2.43.0

Re: [PATCH v2] printk: increase the ring buffer for more than 16 CPUs by default

[John Ogness]

On 2026-09-27, Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> The default LOG_BUF_SHIFT is 17, which means 128 KB, and
> LOG_CPU_MAX_BUF_SHIFT is 12, which means 4 KB per CPU.
>
> By default the ring buffer is increased when
> (num_possible_cpus() - 1) * 4 KB > 64 KB, which requires more than 17
> CPUs.
>
> Remove the explicit subtraction of one, so the ring buffer is increased
> for more than 16 CPUs, which is a power of two. Also fix the outdated
> comment.
>
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>

Reviewed-by: John Ogness <john.ogness@linutronix.de>