Re: [debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

[Arne Jansen]

On 06.06.2011 18:17, Ingo Molnar wrote:
> * Peter Zijlstra<peterz@xxxxxxxxxxxxx>  wrote:
>
> > On Mon, 2011-06-06 at 18:08 +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra<peterz@xxxxxxxxxxxxx>  wrote:
> > >
> > > > On Mon, 2011-06-06 at 17:52 +0200, Ingo Molnar wrote:
> > > > > * Peter Zijlstra<peterz@xxxxxxxxxxxxx>  wrote:
> > > > >
> > > > > > Needs more staring at, preferably by someone who actually
> > > > > > understands that horrid mess :/ Also, this all still doesn't make
> > > > > > printk() work reliably while holding rq->lock.
> > > > >
> > > > > So, what about my suggestion to just *remove* the wakeup from there
> > > > > and use the deferred wakeup mechanism that klogd uses.
> > > > >
> > > > > That would make printk() *visibly* more robust in practice.
> > > >
> > > > That's currently done from the jiffy tick, do you want to effectively
> > > > delay releasing the console_sem for the better part of a jiffy?
> > >
> > > Yes, and we already do it in some other circumstances.
> >
> > We do?
>
> Yes, see the whole printk_pending logic, it delays:
>
>                  wake_up_interruptible(&log_wait);
>
> to the next jiffies tick.
>
> > > Can you see
> > > any problem with that? klogd is an utter slowpath anyway.
> >
> > but console_sem isn't klogd. We delay klogd and that's perfectly
> > fine, but afaict we don't delay console_sem.
>
> But console_sem is really a similar special case as klogd. See, it's
> about a *printk*. That's rare by definition.
>
> If someone on the console sees it he'll be startled by at least 10
> msecs ;-) So delaying the wakeup to the next jiffy really fits into
> the same approach as we already do with&log_wait, hm?

As long as it doesn't scramble the order of the messages, the delay
imho doesn't matter even in very printk-heavy debugging sessions.

> This would solve a real nightmare that has plagued us ever since
> printk() has done wakeups directly - i.e. like forever.
>
> Thanks,
>
> 	Ingo

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