A strange behavior of sched_fair

[Kei Tokunaga]

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)
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