Hi Josh,
On Thu, 12 Jul 2007 10:13:57 -0700 Josh Triplett <[email protected]> wrote:
> On Thu, 2007-07-12 at 09:41 -0700, Josh Triplett 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.
>
> Hmmm, seems I spoke a bit too soon; due to a bug in the test log
> checker, the test failed but the log checker said PASS. Actual results:
> $ grep 'Final ball position' rt-tests.log | sort | uniq -c
> 960 Final ball position: 0
> 39 Final ball position: 1
> 1 Final ball position: 2
>
> So it failed 4% of the runs. However, it failed much less
> spectacularly; rather than overrunning the integer maximum, it only
> reached 1-2. Still a huge improvement despite not solving the problem
> completely.
Yeah, I had a 5000 iterations test run last night which failed with
one of the runs having a final ball position of 1. So it's not yet
perfect. I think we're hitting the same bug as with the O(1) scheduler
in 2.6.21-rt8.
It seems that the more CPUs there are, the easier it is to reproduce.
Hopefully I should have access to a quad dual core box soon now to
verify this.
One more thing, the bug seems even harder to reproduce if I have
a logdev LD_MARK (direct marker, no kprobe) in context_switch().
Ingo, Thomas, any ideas, opinions? It looks like there might be a race in
the RT balancing code that prevents pulling the highest prio task and
instead selects a lower one to run.
In the mean time I'll continue poring over the 2.6.21-rt8 scheduler
to try to find something.
Sébastien.
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