When a task in a taskgroup sleeps, pick_next_task starts all the way back at
the root and picks the task/taskgroup with the min vruntime across all
runnable tasks.

But when there are many frequently sleeping tasks across different taskgroups,
it makes better sense to stay with same taskgroup for its slice period (or
until all tasks in the taskgroup sleeps) instead of switching cross taskgroup
on each sleep after a short runtime.

This helps specifically where taskgroups corresponds to a process with
multiple threads. The change reduces the number of CR3 switches in this case.

Example:

Two taskgroups with 2 threads each which are running for 2ms and
sleeping for 1ms. Looking at sched:sched_switch shows:

BEFORE: taskgroup_1 threads [5004, 5005], taskgroup_2 threads [5016, 5017]
      cpu-soaker-5004  [003]  3683.391089
      cpu-soaker-5016  [003]  3683.393106
      cpu-soaker-5005  [003]  3683.395119
      cpu-soaker-5017  [003]  3683.397130
      cpu-soaker-5004  [003]  3683.399143
      cpu-soaker-5016  [003]  3683.401155
      cpu-soaker-5005  [003]  3683.403168
      cpu-soaker-5017  [003]  3683.405170

AFTER: taskgroup_1 threads [21890, 21891], taskgroup_2 threads [21934, 21935]
      cpu-soaker-21890 [003]   865.895494
      cpu-soaker-21935 [003]   865.897506
      cpu-soaker-21934 [003]   865.899520
      cpu-soaker-21935 [003]   865.901532
      cpu-soaker-21934 [003]   865.903543
      cpu-soaker-21935 [003]   865.905546
      cpu-soaker-21891 [003]   865.907548
      cpu-soaker-21890 [003]   865.909560
      cpu-soaker-21891 [003]   865.911571
      cpu-soaker-21890 [003]   865.913582
      cpu-soaker-21891 [003]   865.915594
      cpu-soaker-21934 [003]   865.917606

Similar problem is there when there are multiple taskgroups and say a task A
preempts currently running task B of taskgroup_1. On schedule, pick_next_task
can pick an unrelated task on taskgroup_2. Here it would be better to give some
preference to task B on pick_next_task.

A simple (may be extreme case) benchmark I tried was tbench with 2 tbench
client processes with 2 threads each running on a single CPU. Avg throughput
across 5 50 sec runs was:

 BEFORE: 105.84 MB/sec
 AFTER:  112.42 MB/sec
Signed-off-by: NVenkatesh Pallipadi <venki@google.com>
Acked-by: NRik van Riel <riel@redhat.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1302802253-25760-1-git-send-email-venki@google.comSigned-off-by: NIngo Molnar <mingo@elte.hu>