Messages in this thread | | | Date | Wed, 18 Jun 2008 17:31:25 +0200 | From | "Daniel K." <> | Subject | Starvation of one RT task when the runtime of another exceeds period. |
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I will demonstrate how to get an RT task stuck, and not rescheduled by (ab)using cgroups and RT scheduling. This is on a 4 core system running 2.6.26-rc6 with two patches applied to make it work at all.
http://marc.info/?i=1213732878.3223.95.camel@lappy.programming.kicks-ass.net http://marc.info/?i=1213789854.16944.216.camel@twins
mkdir /dev/cgroup mount -t cgroup -o cpu,cpuset cgroup /dev/cgroup
# Set up cgroup 0 mkdir /dev/cgroup/0 echo 3 > /dev/cgroup/0/cpuset.cpus echo 0 > /dev/cgroup/0/cpuset.mems echo 100000 > /dev/cgroup/0/cpu.rt_period_us echo 5000 > /dev/cgroup/0/cpu.rt_runtime_us
# Set up cgroup 1 mkdir /dev/cgroup/1 echo 3 > /dev/cgroup/1/cpuset.cpus echo 0 > /dev/cgroup/1/cpuset.mems echo 100000 > /dev/cgroup/1/cpu.rt_period_us echo 5000 > /dev/cgroup/1/cpu.rt_runtime_us
# Start task 1, and assign it to cgroup 0 schedtool -R -p 1 -e burnP6 & [1] 3309 echo 3309 > /dev/cgroup/0/tasks
At this point task 1 use 20% CPU.
# Start task 2, and assign it to cgroup 1 schedtool -R -p 1 -e burnP6 & [2] 3313 echo 3313 > /dev/cgroup/1/tasks
At this point task 2 use 20% CPU. Both tasks use 40% of CPU core#3 in total.
# Assign an insane amount of runtime (over 100%, ref. my other mail) echo 30000 > /dev/cgroup/1/cpu.rt_runtime_us
Now, task 2 use 100% of the CPU, and completely starves task 1, which ceases to get scheduled.
# Cut down on the insanity echo 5000 > /dev/cgroup/1/cpu.rt_runtime_us
Now task 2 use only 20% of the CPU again, task 1 does still not get scheduled.
Let's call this state 'stuck'
I can make task 1 get unstuck by assigning its PID to another cgroup.
# Kick task 1, so it gets scheduled again. echo 3309 > /dev/cgroup/1/tasks
Assuming we go back to state 'stuck', a 'killall burnP6' will only kill task 2, task 1 is still waiting for someone to come and kick it in the butt. As soon as that happens, it will get killed as well.
One time even both tasks got stuck and did not get scheduled, and I needed to kick both tasks to get them going again.
Well, this wasn't really a question, but I'm sure this is not how it's supposed to behave?
Daniel K.
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