Re: [RFC] printk: allow increasing the ring buffer depending on the number of CPUs

[Davidlohr Bueso]

On Wed, 2014-06-11 at 11:34 +0200, Petr MlÃdek wrote:
> On Tue 2014-06-10 18:04:45, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> > 
> > 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@xxxxxxx>
> > Cc: Petr Mladek <pmladek@xxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Joe Perches <joe@xxxxxxxxxxx>
> > Cc: Arun KS <arunks.linux@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
> > ---
> >  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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