Date: Mon, 6 Jun 2011 18:17:49 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Arne Jansen <lists@die-jansens.de>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
        hpa@zytor.com, linux-kernel@vger.kernel.org, efault@gmx.de,
        npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com,
        tglx@linutronix.de, linux-tip-commits@vger.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
Message-ID: <20110606161749.GA22157@elte.hu>
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 <20110606145827.GD30348@elte.hu>
 <1307372989.2322.136.camel@twins>
 <1307375227.2322.161.camel@twins>
 <20110606155236.GA7374@elte.hu>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2011-06-06 at 18:08 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > On Mon, 2011-06-06 at 17:52 +0200, Ingo Molnar wrote:
> > > > * Peter Zijlstra <peterz@infradead.org> 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?

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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