linux-kernel - Re: deadlock in scheduler enabling HRTICK feature

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Date:	Thu, 27 Jun 2013 14:28:01 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	David Ahern <dsahern@...il.com>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, tglx@...utronix.de
Subject: Re: deadlock in scheduler enabling HRTICK feature

On Thu, 2013-06-27 at 12:53 +0200, Peter Zijlstra wrote: 
> On Thu, Jun 27, 2013 at 12:43:09PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 26, 2013 at 10:46:33AM -0600, David Ahern wrote:
> > > On 6/26/13 1:05 AM, Peter Zijlstra wrote:
> > > >>What is the expectation that the feature provides? not a whole lot of
> > > >>documentation on it. I walked down the path wondering if it solved an odd
> > > >>problem we are seeing with the CFS in 2.6.27 kernel.
> > > >
> > > >Its supposed to use hrtimers for slice expiry instead of the regular tick.
> > > 
> > > So theoretically CPU bound tasks would get preempted sooner? That was my
> > > guess/hope anyways.
> > 
> > Doth the below worketh?
> > 
> 
> Related to all this; the reason its not enabled by default is that mucking
> about with hrtimers all the while is god awful expensive.

That cost sprang to mind when you mentioned that deadline needs hrtick.

> I've had ideas about making this a special purpose 'hard-coded' timer in the
> hrtimer guts that's only ever re-programmed when the new value is sooner.
> 
> By making it a 'special' timer we can avoid the whole rb-tree song and dance;
> and by taking 'spurious' short interrupts we can avoid prodding the hardware
> too often.
> 
> Then again; Thomas will likely throw frozen seafood my way for even proposing
> stuff like this and I'm not even sure that's going to be enough to make the
> cost acceptable.

Could be worse, flaming marshmallows stick.

-Mike

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