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SubjectRe: [RFC] printk: allow increasing the ring buffer depending on the number of CPUs
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On Wed, 2014-06-11 at 11:34 +0200, Petr Mládek wrote:
> On Tue 2014-06-10 18:04:45, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@suse.com>
> >
> > The default size of the ring buffer is too small for machines
> > with a large amount of CPUs under heavy load. What ends up
> > happening when debugging is the ring buffer overlaps and chews
> > up old messages making debugging impossible unless the size is
> > passed as a kernel parameter. An idle system upon boot up will
> > on average spew out only about one or two extra lines but where
> > this really matters is on heavy load and that will vary widely
> > depending on the system and environment.
>
> Thanks for looking at this. It is a pity to lose stracktrace when a huge
> machine Oopses just because the default ring buffer is too small.

Agreed, I would very much welcome something like this.

> > There are mechanisms to help increase the kernel ring buffer
> > for tracing through debugfs, and those interfaces even allow growing
> > the kernel ring buffer per CPU. We also have a static value which
> > can be passed upon boot. Relying on debugfs however is not ideal
> > for production, and relying on the value passed upon bootup is
> > can only used *after* an issue has creeped up. Instead of being
> > reactive this adds a proactive measure which lets you scale the
> > amount of contributions you'd expect to the kernel ring buffer
> > under load by each CPU in the worst case scenerio.
> >
> > We use num_possible_cpus() to avoid complexities which could be
> > introduced by dynamically changing the ring buffer size at run
> > time, num_possible_cpus() lets us use the upper limit on possible
> > number of CPUs therefore avoiding having to deal with hotplugging
> > CPUs on and off. This option is diabled by default, and if used
> > the kernel ring buffer size then can be computed as follows:
> >
> > size = __LOG_BUF_LEN + (num_possible_cpus() - 1 ) * __LOG_CPU_BUF_LEN
> >
> > Cc: Michal Hocko <mhocko@suse.cz>
> > Cc: Petr Mladek <pmladek@suse.cz>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Joe Perches <joe@perches.com>
> > Cc: Arun KS <arunks.linux@gmail.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
> > ---
> > init/Kconfig | 28 ++++++++++++++++++++++++++++
> > kernel/printk/printk.c | 6 ++++--
> > 2 files changed, 32 insertions(+), 2 deletions(-)
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 9d3585b..1814436 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -806,6 +806,34 @@ config LOG_BUF_SHIFT
> > 13 => 8 KB
> > 12 => 4 KB
> >
> > +config LOG_CPU_BUF_SHIFT
> > + int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)"
> > + range 0 21
> > + default 0
> > + help
> > + The kernel ring buffer will get additional data logged onto it
> > + when multiple CPUs are supported. Typically the contributions is a
> > + few lines when idle however under under load this can vary and in the
> > + worst case it can mean loosing logging information. You can use this
> > + to set the maximum expected mount of amount of logging contribution
> > + under load by each CPU in the worst case scenerio. Select a size as
> > + a power of 2. For example if LOG_BUF_SHIFT is 18 and if your
> > + LOG_CPU_BUF_SHIFT is 12 your kernel ring buffer size will be as
> > + follows having 16 CPUs as possible.
> > +
> > + ((1 << 18) + ((16 - 1) * (1 << 12))) / 1024 = 316 KB
>
> It might be better to use the CPU_NUM-specific value as a minimum of
> the needed space. Linux distributions might want to distribute kernel
> with non-zero value and still use the static "__log_buf" on reasonable
> small systems.

It should also depend on SMP and !BASE_SMALL.
I was wondering about disabling this by default as it would defeat the
purpose of being a proactive feature. Similarly, I worry about distros
choosing a correct default value on their own.

> > + Where as typically you'd only end up with 256 KB. This is disabled
> > + by default with a value of 0.
>
> I would add:
>
> This value is ignored when "log_buf_len" commandline parameter
> is used. It forces the exact size of the ring buffer.

... and update Documentation/kernel-parameters.txt to be more
descriptive about this new functionality.

> > + Examples:
> > + 17 => 128 KB
> > + 16 => 64 KB
> > + 15 => 32 KB
> > + 14 => 16 KB
> > + 13 => 8 KB
> > + 12 => 4 KB
>
> I think that we should make it more cleat that it is per-CPU here,
> for example:
>
> 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
>

Agreed.

> > #
> > # Architectures with an unreliable sched_clock() should select this:
> > #
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 7228258..2023424 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -246,6 +246,7 @@ static u32 clear_idx;
> > #define LOG_ALIGN __alignof__(struct printk_log)
> > #endif
> > #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
> > +#define __LOG_CPU_BUF_LEN (1 << CONFIG_LOG_CPU_BUF_SHIFT)
> > static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
> > static char *log_buf = __log_buf;
> > static u32 log_buf_len = __LOG_BUF_LEN;
> > @@ -752,9 +753,10 @@ void __init setup_log_buf(int early)
> > unsigned long flags;
> > char *new_log_buf;
> > int free;
> > + int cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_BUF_LEN;

If depending on SMP, you can remove the - 1 here.

> > - if (!new_log_buf_len)
> > - return;
> > + if (!new_log_buf_len && cpu_extra > 1)
> > + new_log_buf_len = __LOG_BUF_LEN + cpu_extra;
>
> We still should return when both new_log_buf_len and cpu_extra are
> zero and call here:
>
> if (!new_log_buf_len)
> return;

Yep.

> Also I would feel more comfortable if we somehow limit the maximum
> size of cpu_extra. I wonder if there might be a crazy setup with a lot
> of possible CPUs and possible memory but with some minimal amount of
> CPUs and memory at the boot time.

Maybe. But considering that systems with a lot of CPUs *do* have a lot
of memory, I wouldn't worry much about this, just like we don't worry
about it now. Considering a _large_ 1024 core system and using the max
value 21 for CONFIG_LOG_BUF_SHIFT, we would only allocate just over 2Gb
of extra space -- trivial for such a system. And if it does break
something, then heck, go fix you box and/or just reduce the percpu
value. I guess that's a good reason to keep the default to 0 and let
users play with it as they wish without compromising uninterested
parties. afaict only x86 would be exposed to systems not booting if we
fail to allocate.

Thanks,
Davidlohr



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