Messages in this thread | | | Date | Thu, 15 Dec 2011 19:07:25 +0100 | From | Ingo Molnar <> | Subject | Re: printk() vs tty_io |
| |
* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Thu, Dec 15, 2011 at 1:56 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > > > We probably could, I can have a closer look, but the main question is, > > are we going to commit to no wakeups from console implementations? That > > would mean removing the USB serial console support and other such stuff. > > I guess we can't. > > So new approach: > > - screw it. the rq lock is just too central, you cannot call > printk from under it. And if you have an oops under it, you're > screwed. > > Peter, why do you want to try to work from under the rq > lock?
Well, no real strong reason: we had that nasty xtime_lock related lockup in printk() (which was arguably much worse than the rq lock dependency - and it got fixed), and everyone assumed that somehow we could just remove the rq lock dependency from printk() as well.
It appears we can't - and that's OK.
Was an intellectual excercise which didnt work out.
> - Make a special "debug printk" that is not synchronous. Just > make it buffer things, and have it actually print out from a > worker thread or whatever. This one *only* takes the lock for > that buffer itself, and works everywhere. We could probably > even do tricks to make it NMI-safe.
We kind of already have such a thing: trace_printk() - which works from the weirdest of atomic contexts as well.
IIRC Peter uses trace_printk() to develop the scheduler all the time.
What we could perhaps do is a sort of 'shut up regular printk and feed all printks into the trace buffers' kind of debug switch.
> Make code that isn't an oops or other very synchronous > "have to print out *now*" aim to use this "softer" printk.
For early and nast oopses we have a very primitive printk: earlyprintk=keep which can just act as a full printk replacement. I use earlyprintk=keep while regular printk is disabled (there's no console).
[ earlyprintk can be used to debug everything except printk() lockups, obviously. ]
Thanks,
Ingo
| |