Question about group scheduler cpu shares

From: Gaurav Poothia <gaurav.poothia@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: Question about group scheduler cpu shares
Date: Thu, 21 Jul 2016 12:03:47 -0700	[thread overview]
Message-ID: <CADDfC=q_AkomUqKShkF=hkwjB2Tu-WMiNmfhUEPWVy1gr_xFLQ@mail.gmail.com> (raw)

Hi,
The cgroup "cpu" subsystem's weight calculation using "cpu shares" is
fairly clear when all tasks are attached to the leaf cgroups in "cpu"
subsystem.
For example in this cgroup hierarchy with tasks A.B,C and D and
associated cpu shares:

ROOT
|
+ -Group1(3072)
|    |
|   +- A(2048)
|    |
|    +- B(1024)
|
+- Group2(2048)
     |
     +-C(1024)
     |
     +-D(1024)

Group 1 and Group 2 split the CPU 60% to 40% respectively
A and B split Group1's 60% share 2/3rd  to 1/3rd so absolute share of
CPU will be 40% and 20% respectively
C and D split Group2's 40% share evenly so absolute shares or 20% each
So far so good. But in this example all tasks are associated with leaf cgroups.

Say I add a task E to Group1's task list (note that is an interior aka
non-leaf node)
How does the CPU split change between A, B and E.
AFAICT there is no cgroup cpu subsystem knob to weight tasks on an
interior node against the tasks in that node's children

Thanks in advance!
Gaurav