Android: automated per-uid task group

Android is a privilege-separated operating system, in which each application runs with a distinct system identity: the Linux user ID (uid).

With this patch (sched-automated-per-uid-task-groups.patch) the kernel automatically creates a distinct task group for each uid (when a process calls set_user()) and places all the tasks that belong to a single uid into the same task group. In this way each application can get a fair amount of the CPU bandwidth (guaranteed by the CFS scheduler), independently by the number of task/threads spawned.

The patch is against the CyanogenMod's 6.1.1 kernel (2.6.35.10) and I tested it successfully on my HTC Desire (Bravo).

I've used the following testcase:

- run 4 cpu hogs in background as user app_35 (com.android.email in my case):

$ for i in `seq 4`; do yes >/dev/null & done

- run the Quadrant benchmark in parallel and measure the result with and without the patch applied

Without the patch (output of top):

PID PPID USER STAT VSZ %MEM CPU %CPU COMMAND

6533 123 10070 R 202m 48.6 0 20.0 com.aurorasoftworks.quadrant.ui.st

6506 1 10035 R 1128 0.2 0 20.0 yes

6507 1 10035 R 1128 0.2 0 20.0 yes

6508 1 10035 R 1128 0.2 0 20.0 yes

6509 1 10035 R 1128 0.2 0 20.0 yes

Benchmark result: 676

uid 10035 (cpu hog) : 60.0 % cpu quota

uid 10070 (benchmark): 20.0 % cpu quota

With automated per-uid task group (output of top):

PID PPID USER STAT VSZ %MEM CPU %CPU COMMAND

6784 123 10070 S 209m 51.4 0 50.0 com.aurorasoftworks.quadrant.ui.st

6852 1 10035 R 1128 0.2 0 12.5 yes

6853 1 10035 R 1128 0.2 0 12.5 yes

6854 1 10035 R 1128 0.2 0 12.5 yes

6855 1 10035 R 1128 0.2 0 12.5 yes

Benchmark result: 816

uid 10035 (cpu hog) : 50.0 % cpu quota

uid 10070 (benchmark): 50.0 % cpu quota

Total speedup: ~1.2 (the benchmark is about 20% faster in this case), because in the last case the benchmark gets ~50% of the CPU time and in the other case it gets only ~20%, despite the fact that there are 2 "pair" applications that should be correctly considered as equal from the user's perspective.