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

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Date:	Thu, 15 Mar 2012 17:04:31 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Mandeep Singh Baines <msb@...omium.org>
Subject: Re: [PATCH] watchdog: Make sure the watchdog thread gets CPU on
 loaded system

On Thu, 2012-03-15 at 11:54 -0400, Don Zickus wrote:
> > > Why did watchdog() reset the scheduling policy seven instructions
> > > before exiting?  Seems pointless.
> > 
> > It has been introduced by Thomas in cba9bd22. To be honest I don't
> > understand why it makes a sense?
> 
> Yeah I noticed that too.  I didn't bother questioning it either when
> it
> went in.  I just assumed Thomas and Peter know scheduling a lot better
> than I do. :-) 

I just dug through my IRC logs and the reason is that running the entire
exit path as a highest priority RT task incurs a latency spike to other
tasks running on the system (313us). Since there's absolutely no point
in running the exit path as a RT task, tglx made it not do that.
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