linux-kernel - Re: [PATCH] watchdog: Make sure the watchdog thread gets CPU on loaded system

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Message-ID: <20120315124228.GA5318@elte.hu>
Date:	Thu, 15 Mar 2012 13:42:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] watchdog: Make sure the watchdog thread gets CPU on
 loaded system


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2012-03-15 at 12:00 +0100, Peter Zijlstra wrote:
> > On Wed, 2012-03-14 at 18:45 -0700, Mandeep Singh Baines wrote:
> > > You could make MAX_RT_PRIO greater than MAX_USER_RT_PRIO but that
> > > might
> > > have some impact on real-time applications. A simple one-line patch:
> > > 
> > > - #define MAX_RT_PRIO           MAX_USER_RT_PRIO
> > > + #define MAX_RT_PRIO           (MAX_USER_RT_PRIO + 1)
> > > 
> > > would prevent user-space from causing a false lockup detection.
> > 
> > We're so not going to muck with the fifo priorities just for this stupid
> > soft watchdog,.. I already hate that I can't disable the piece of crap,
> > making it more involved is just really not going to happen.
> 
> And before people start to whinge about that, all the soft 
> watchdog issues I've seen fly by the past year or so all were 
> bugs in the watchdog itself, I can't actually remember it 
> flagging a real problem.

Its efficiency always depended on which area I was working on. 
For syscall level stuff it helped me numerous times.

> The NMI watchdog otoh works like a charm for me and regularly 
> helps out when I done stupid.

Sure, you are mostly working on perf events, the scheduler and 
related core kernel areas so when you are stupid you get a hard 
lockup or worse, quickly. Not much room for soft lockups.

So it's more of a case of selection bias, me thinks.

So unless there's concensus to remove everything but the hard 
lockup detection facilities, lets solve the technical problem at 
hand, ok?

Thanks,

	Ingo
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