Re: [PATCH 5/5] ring-buffer: fix printk output

Ingo Molnar
2009-04-29T20:11:37Z

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 29 Apr 2009 21:45:46 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > > On Wed, 29 Apr 2009 11:56:25 +0200 Ingo Molnar <mingo@elte.hu> wrote:
> > >
> > > >
> > > > > > My larger point remains, about possibly embedding linux-next
> > > > > > into lkml. I couldnt think of a single linux-next mail that isnt
> > > > > > relevant to lkml. It's all about commits that are destined for
> > > > > > upstream in 0-2.5 months.
> > > > >
> > > > > Sure, I'd be OK with zapping the linux-next list.
> > > >
> > > > Another, less drastic solution would be to keep it as an _alias_
> > > > list. All mails posted to it also go to lkml, but it would still be
> > > > subscribe-able separately.
> > >
> > > That would work, although I wonder about the potential for
> > > duplicates turning up somewhere.
> >
> > The potential for duplicates is inherent in Cc: lines to begin with.
>
> But if someone does reply-to-all to linux-next and linux-kernel,
> and the linux-next email gets redirected to linux-kernel, badness
> might happen. Bearing in mind the screwiness of the mail clients
> whcih some people use..

Hm, what badness would happen? Today, if i send a mail to linux-next
and lkml, it shows up on both lists. If i'm subscribed to both, i
get two mails - one from lkml and one from linux-next.

Auto-aliasing the lists means adding an implicit Cc: lkml to all
mails that the linux-next list server gets. Two mails are generated
- and if someone is subscribed to both lists, two mails are
received. How would mail clients have a problem with that? It's
already happening.

> > > > ( This has come up before and this would be useful for a number of
> > > > other things - such as tracing/instrumentation. Someone who is
> > > > only interested in instrumentation related discussions could
> > > > subscribe to that list. )
> > > >
> > > > > > > printk_once() is racy on smp and preempt btw ;)
> > > > > >
> > > > > > Like WARN_ONCE() and WARN_ON_ONCE(). It's really an "oh crap"
> > > > > > facility, not for normal kernel messages.
> > > > > >
> > > > > > Do we want to complicate them with locking and preemption - or
> > > > > > should we just concentrate on getting the "oh crap" message out
> > > > > > to the syslog (before it's possibly too late to get anything
> > > > > > out)?
> > > > > >
> > > > > > I have no strong opinion about it - but i tend to like the
> > > > > > simpler method most. printk + stack dumps themselves arent
> > > > > > atomic to begin with.
> > > > >
> > > > > Well, it's hardly likely to be a problem. otoh, if two CPUs _do_
> > > > > hit the thing at the same time, the resulting output will be all
> > > > > messed up and we'd really like to see it.
> > > > >
> > > > > Easily fixed with test_and_set_bit()?
> > > >
> > > > but if two CPUs hit it at once then the printk+stack-dump itself is
> > > > already mixed up. So if we do any atomicity it should be done for
> > > > all the print-once APIs. (note, lockdep does such message-atomicity
> > > > already, in its own facility)
> > >
> > > Confused.
> > >
> > > <gets distracted by FW_BUG and friends. ytf are they in kernel.h?>
> > >
> > > #define printk_once(x...) ({ \
> > > static unsigned long __print_once; \
> > > \
> >
> > hm, this doubles the flag size on 32-bit kernels.
>
> Well yeah. I was wondering whether __print_once should be a char
> anyway. Will that hurt text size on any arch? Will gcc dtrt with
> such things? It might go and 4-byte align the chars anwyay.

a char would make sense - since they are all rare codepaths
compression is important - and this should be done for the other
_ONCE APIs as well.

gcc does the right thing and will compress adjacent char's - but the
problem is that these char's are unlikely to be adjacent - te
adjacent variables will likely be larger so the chars will take up 4
or 8 bytes in practice.

So to achieve compression we'd have to put them into a separate data
section and have a per arch linker script detail for that. Is that
worth the trouble? How many flags are we talking about?

Ingo