A strange behavior of sched_fair

[Kei Tokunaga <tokunaga.keiich@jp.fujitsu.com>]

Hi Ingo,

I am playing around with sched_fair and cgroup, and it seems like
I hit a possible bug.  Could you also check if that is a bug?

Description of behavior:
   Start a cpu-bound task (t1), attach it to a cgroup (cgA), and let the
   task to run for a while (e.g. several tens of seconds or a couple of
   minutes would be adequate.)  Then, start another cpu-bound task (t2)
   and attach it to cgA in the way described in "Steps to Reproduce" section.
   You will see t1 does not get run for a while.
   (The tasks may not have to be cpu-bound, but it is easier to see the
    behavior using cpu-bound tasks.)

How reproducible:
   Always.

Environments where I saw the behavior:
   2.6.25-rc3 with resource management functions enabled on ia64 box.

Steps to Reproduce:
   # mkdir /dev/cgroup
   # mount -t cgroup -ocpuset,cpu cpu /dev/cgroup
   # mkdir /dev/cgroup/{a,b}
   # echo 0 > /dev/cgroup/a/cpuset.cpus
   # echo 0 > /dev/cgroup/b/cpuset.cpus
   # echo 1 > /dev/cgroup/a/cpuset.mems
   # echo 1 > /dev/cgroup/b/cpuset.mems
   # echo $$ > /dev/cgroup/b/tasks
   # ./a.out & echo $! > /dev/cgroup/a/tasks (a.out is just a for-loop program)
     [Wait for several tens of seconds or a couple of minutes.]
   # ./a.out2 & echo $! > /dev/cgroup/a/tasks (a.out2 is just a for-loop program)
     [You will see a.out does not get run for a while by running top command.]

Additional Info:
   a.out2 needs to be started from the shell of cgroup-b in order to
   reproduce the problem (, unless the system is UP.)  Starting a.out2
   in the manner, se->vruntime (or something to create the se->vruntime)
   of a.out2 seems to be initialized to a small value, compared to the
   value of a.out.  And the fair scheduler only runs a.out2 until the
   se->vruntime catches up with the se->vruntime of a.out.

Thanks,
Kei
--
Kei Tokunaga
Fujitsu (Red Hat On-site Partner)