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

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Message-ID: <20130627130609.GA6655@gmail.com>
Date:	Thu, 27 Jun 2013 15:06:09 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	David Ahern <dsahern@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, tglx@...utronix.de
Subject: Re: deadlock in scheduler enabling HRTICK feature


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

Sounds neat ...

> 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: rotten seafood?)

Thanks,

	Ingo
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