Re: deadlock in scheduler enabling HRTICK feature

[Peter Zijlstra]

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.

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.
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