Re: [PATCH] printk: fix wrong number of CPUs required to increase the ring buffer

[Sang-Heon Jeon]

Hi,

2026년 9월 23일 (수) 오후 8:57, Petr Mladek <pmladek@xxxxxxxx>님이 작성:
>
> On Tue 2026-09-22 01:38:27, Sang-Heon Jeon 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 18 or more CPUs.
>
> Good catch!

Thanks for the review, Petr.

> > Fix the wrong comment and help text.
> >
> > Signed-off-by: Sang-Heon Jeon <ekffu200098@xxxxxxxxx>
> > ---
> >  init/Kconfig           | 2 +-
> >  kernel/printk/printk.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 60448fd4ee12..446cbb59a43b 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -843,7 +843,7 @@ 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 16 CPUs are needed to trigger the allocation.
> > +       so that 18 or more CPUs are needed to trigger the allocation.
>
> The number 18 is pretty ugly. It was supposed to be a power of 2.
> I would prefer to fix the computation instead.

I agree. If possible, I also prefer that too.

> >
> >         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.
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 00f8e1291564..c7d961256c88 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1087,7 +1087,7 @@ static void __init log_buf_add_cpu(void)
> >
> >       cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MAX_BUF_LEN;
>
> If we remove the minus one:
>
>         cpu_extra = num_possible_cpus() * __LOG_CPU_MAX_BUF_LEN;
>
> The it should be > 16 and we could keep init/Kconfig as is.
>
> > -     /* by default this will only continue through for large > 64 CPUs */
> > +     /* by default this will only continue through for 18 or more CPUs */
>
> Right, this should be > 16 with the current defaults.

Ditto.

> >       if (cpu_extra <= __LOG_BUF_LEN / 2)
> >               return;
>
> Would you like to prepare v2?

Sure, I'll send the v2 patch soon.

> Best Regards,
> Petr

Best regards,
Sang-Heon Jeon