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SubjectRe: [Patch RT] Fix CFS load balancing for RT tasks

Hi Josh,

On Thu, 12 Jul 2007 09:41:55 -0700 Josh Triplett <josht@linux.vnet.ibm.com> wrote:

> On Wed, 2007-07-11 at 16:47 +0200, Sébastien Dugué wrote:
> > there seems to be something wrong with the way the CFS balances (or does not
> > balance) RT tasks. This was evidenced using the sched_football testcase
> > available from the RT wiki (http://rt.wiki.kernel.org/index.php/IBM_Test_Cases)
> > which I modified and attached to this mail.
> >
> > The testcase starts a number of threads which fall into 3 categories:
> >
> > 1 referee thread: SCHED_FIFO, RT prio 5
> > ncpus defensive threads: SCHED_FIFO, RT prio 4
> > ncpus offensive threads: SCHED_FIFO, RT prio 3
> >
> > (ncpus being the number of CPUs)
> >
> > To make a long story short, the defensive threads should end up distributed
> > among all CPUs, but that's not the case. For example, on a dual HT Xeon box,
> > after task migration stabilizes we have the following running on the different
> > CPUs:
> >
> > CPU 0: defense2
> > CPU 1: referee offense2 offense3 offense4 defense3
> > CPU 2: offense1
> > CPU 3: defense1 defense4
> >
> > which clearly show the imbalance between CPU 2 and CPU 3 where offense1
> > should not be allowed to run while the higher prio defense1 and defense4
> > are sharing the same CPU.
> >
> > The following patch fixes this by re-enabling the RT overload detection
> > for the CFS. It may not be the right solution, maybe it should be incorporated
> > into the other load balancing mechanisms. I did not digg deep enough yet
> > to make that call ;-)
>
> 2.6.21.5-rt20 plus this patch passed 1000 runs of the standard
> sched_football on an 8 processor (quad dual-core) x86-64 box. Nice
> work.

Thanks, but a nightly run revealed that there is still a bug. At least
now CFS is on par with O(1).


>
> > The RT overload mechanism of the O(1) scheduler has not been activated
> > in the new CFS.
> >
> > This patch fixes that by inserting calls to inc_rt_tasks() and dec_rt_tasks()
> > in enqueue_task_rt() and dequeue_task_rt() respectively, which enables the
> > balance_rt_tasks() to be run in the rt_overload case.
> >
> >
> > Signed-off-by: Sébastien Dugué <sebastien.dugue@bull.net>
>
> Acked-by: Josh Triplett <josh@kernel.org>
>
> - Josh Triplett

Sébastien.
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