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SubjectRe: [PATCH printk 3/5] printk: use buffer pool for sprint buffers
On 30/09/2020 15.35, Steven Rostedt wrote:
> On Wed, 30 Sep 2020 10:06:24 +0200
> Rasmus Villemoes <> 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 is a writable file containing four
integer val‐
ues that influence kernel printk() behavior when printing or
error messages. The four values are:

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
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- eth0: ax88179
- Link status is: 1
kern :info : [ 4642.218100] ax88179_178a 3- eth0: ax88179
- Link status is: 0
kern :info : [ 4643.882038] ax88179_178a 3- eth0: ax88179
- Link status is: 1
kern :info : [ 4667.562011] ax88179_178a 3- 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.


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