Re: [PATCH] printk: Add printk_flush() to force buffered text to console

[Joe Perches <joe@perches.com>]

On Sat, 2012-06-23 at 13:47 +0200, Kay Sievers wrote:
[]
> We need make some sort of a compromise for the self-tests here, I guess.
> I think the trade of using one of these options for the self-tests to
> cope with the new model is justified:
> - flush explicitly when it is needed, by a global flag or with a
>   message prefix

> - printing a second full line that says: test foo OK, instead
>   of appending the OK to the former line

This is probably the easiest actual fix.

> - printing one full line, and suppress the later OK entirely. The OK
>   might not be needed at all; we will find out that the machine
>   has not crashed at that point and a message in case of an error
>   could be sufficient in most cases.

I think the added timing information useful.

> Reverting the changes now would remove:
> - proper device identifiers attached to a message, which allows
>   tools for the first time to know the contexts of the kernel messages

I think this is not particularly true nor really useful at
present and the [v]printk_emit parts should be removed
until a the benefit of such a capability more proven.

I'm still concerned someone is going to say that the
because you say these things, someone else is going to
declare that the content of the [v]printk_emit bits is
effectively an ABI (inviolate and immutable) like the
seq_ functions.  I think that'd make changing any of the
logging much more difficult and should be avoided.

> - 100% message integrity of structured messages, and single line
>   prints, and if only one continuation line user prints at a time.
>   We never mix continuation users with full line users, like the
>   old buffer did.
> - very reliable continuation line prints, because of the buffering.
>   The only race that can still happen is several cont users racing
>   against each other, which is not too likely

It's hard to say that it's not too likely.  I think it
depends on cpu count.  More cpus, more likely.

Still, it's decidedly better than it was.

> - 100% granularity at line level during userspace buffer access,
>   crash dumping; we never copy around half or overwritten lines,
>   like the old buffer
> - the ring buffer prunes only entire messages not just the half
>   of a line like the old byte buffer
> - sequence numbers allow us to track the state of the read and write
>   position in the buffer, so we never copy in-the-meantime already
>   overwritten messages around
> - sequence numbers which allow userspace to reconnect to the old reading
>   position and process only new messages, and find if messages got
>   overwritten
> - support for multiple concurrent readers with live update of the kernel
>   log stream

Good features list and it was well implemented too.

I think the uses of the direct emit model should simply
be updated to full lines as you suggested.  There are not
too many files that need touching.