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. > > 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@xxxxxxxx> > > Acked-by: Josh Triplett <josh@xxxxxxxxxx> Ack still valid despite the above. - Josh Triplett - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
