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

[Arne Jansen]

On 06.06.2011 18:38, Arne Jansen wrote:
> 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.

And, as important, doesn't reduce the throughput of printk. Having only
100 wakeups/s sounds like the throughput is limited to 100xsizeof(ring 
buffer).

> > 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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