Re: [Ksummit-discuss] [TECH TOPIC] printk redesign

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 20 Jun 2017 18:27:38 +0100
Mark Brown <broonie@kernel.org> wrote:

> On Wed, Jun 21, 2017 at 02:11:34AM +0900, Sergey Senozhatsky wrote:
> 
> > another thing that I found useful is a CPU number of the processor
> > that stored a particular line to the logbuf.  
> 
> At some point we start reinventing ftrace...  there's issues with
> joining the two up but there should at least be lessons we can learn.

I've thought about this a little too.

I would like printk to have per-cpu buffers. Then we don't even need to
store the CPU number, that would be explicit by which buffer the data
is stored in.

The one thing that is needed, is the consumer. In ftrace, it's whatever
reads the buffer, which is usually user space, but can be the kernel
(see sysctl-z). But there's only one consumer at a time.

I was thinking about a new design for printk. Similar to ftrace, but
different.

1) have per cpu buffers, that are lockless. Writes happen immediately,
but the output happens later.

2) have two types of console interfaces. A normal and a critical.

3) have a thread that is woken whenever there is data in any of the
buffers, and reads the buffers, again lockless. But to do this in a
reasonable manner, unless you break the printks up in sub buffers like
ftrace, if the consumer isn't fast enough, newer messages are dropped.

4) If a critical print is needed (and here's why we have two console
interfaces), the normal console interface gets turned off, and the
buffers stop being output through them. What ever called the critical
print, will take over, and flush out all the contents of the current
buffers. Then anything printed during the critical section will go out
immediately (no buffering). The printk thread, will stop having access
to the buffers, and shutdown till the critical section is complete.

This is just a rough idea. I think it is possible. The tricky part is
going to be the switch over to the critical section. Also, have a
command line parameter that has all printks be critical. Peter Zilstra
has some patches that already does that with making printk turn into
early printk.

-- Steve