On 06/27/2013 03:53 AM, 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. > > 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.
Supposedly, on really new x86 systems, the TSC deadline timer is so fast to reprogram that this isn't worth it. I wonder if the general timing code should have a way to indicate that a given clockevent is (a) very fast and (b) is actually locked to a clocksource as opposed to just having a vaguely accurately calibrated frequency. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
