Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

[Petr Mladek <pmladek@suse.com>]

On Thu 2020-10-01 09:15:39, Rasmus Villemoes wrote:
> On 30/09/2020 15.35, Steven Rostedt wrote:
> > On Wed, 30 Sep 2020 10:06:24 +0200
> > Rasmus Villemoes <rasmus.villemoes@prevas.dk> wrote:
> > 
> >> True. But remember that printk is called from _everywhere_, with all
> >> sorts of locks held and/or preemption disabled or whatnot, and every
> >> cycle spent in printk makes those windows wider. Doubling the cost of
> >> every single printk by unconditionally doing vsnprintf() twice is a bad
> >> idea.
> > 
> > But the console output is usually magnitudes more expensive than the
> > vsnprintf(), would doing it twice really make a difference?
> 
> AFAIU, not every message gets printed to the console directly - syslog(2):
> 
>    /proc/sys/kernel/printk
>        /proc/sys/kernel/printk is a writable file containing four
> integer val‐
>        ues that influence kernel printk() behavior when  printing  or
> logging
>        error messages.  The four values are:
> 
>        console_loglevel
>               Only  messages  with  a  log level lower than this value
> will be
>               printed to the console.  The default value  for  this
> field  is
>               DEFAULT_CONSOLE_LOGLEVEL  (7),  but it is set to 4 if the
> kernel
>               command line contains the word "quiet",
> 
> So the normal state of things is that you don't pay the cost of printing
> to the console for all the pr_debug (ok, they may be compiled out or
> run-time disabled depending on DYNAMIC_DEBUG etc.), nor info, notice,
> warn. For those messages that are not directly written to the console,
> the vsnprintf() is a large part of the cost (not exactly half, of
> course, so doubling is an exaggeration, but whether it's 70% or 100%
> doesn't really matter).
> 
> I'm not at all concerned about pr_err and above becoming more expensive,
> they are rare. But random drivers are filled with random pr_info in
> random contexts - just a small selection from dmesg -x shows these
> really important things:
> 
> kern  :info  : [ 4631.338105] ax88179_178a 3-13.2.3.3:1.0 eth0: ax88179
> - Link status is: 1
> kern  :info  : [ 4642.218100] ax88179_178a 3-13.2.3.3:1.0 eth0: ax88179
> - Link status is: 0
> kern  :info  : [ 4643.882038] ax88179_178a 3-13.2.3.3:1.0 eth0: ax88179
> - Link status is: 1
> kern  :info  : [ 4667.562011] ax88179_178a 3-13.2.3.3:1.0 eth0: ax88179
> - Link status is: 0
> ...
> kern  :info  : [ 9149.215456] [drm] ring test on 1 succeeded in 1 usecs
> kern  :info  : [ 9149.215459] [drm] ring test on 2 succeeded in 1 usecs
> kern  :info  : [ 9149.215466] [drm] ring test on 3 succeeded in 4 usecs
> 
> and if I'm reading the code correctly, the former is even an example of
> something that happens in irq context.

As I already wrote. We might optimize this when it causes real
life problems. And trace_printk() is a better choice for performance
sensitive debugging.

Best Regards,
Petr