Re: [PATCHSET RFC cgroup/for-4.6] cgroup, sched: implement resource group and PRIO_RGRP

From: Mike Galbraith
Date: Sat Mar 12 2016 - 01:27:19 EST

On Fri, 2016-03-11 at 10:41 -0500, Tejun Heo wrote:
> Hello,
> This patchset extends cgroup v2 to support rgroup (resource group) for
> in-process hierarchical resource control and implements PRIO_RGRP for
> setpriority(2) on top to allow in-process hierarchical CPU cycle
> control in a seamless way.
> cgroup v1 allowed putting threads of a process in different cgroups
> which enabled ad-hoc in-process resource control of some resources.
> Unfortunately, this approach was fraught with problems such as
> membership ambiguity with per-process resources and lack of isolation
> between system management and in-process properties. For a more
> detailed discussion on the subject, please refer to the following
> message.
> [1] [RFD] cgroup: thread granularity support for cpu controller
> This patchset implements the mechanism outlined in the above message.
> The new mechanism is named rgroup (resource group). When explicitly
> designating a non-rgroup cgroup, the term sgroup (system group) is
> used. rgroup has the following properties.
> * A rgroup is a cgroup which is invisible on and transparent to the
> system-level cgroupfs interface.
> * A rgroup can be created by specifying CLONE_NEWRGRP flag, along with
> CLONE_THREAD, during clone(2). A new rgroup is created under the
> parent thread's cgroup and the new thread is created in it.
> * A rgroup is automatically destroyed when empty.
> * A top-level rgroup of a process is a rgroup whose parent cgroup is a
> sgroup. A process may have multiple top-level rgroups and thus
> multiple rgroup subtrees under the same parent sgroup.
> * Unlike sgroups, rgroups are allowed to compete against peer threads.
> Each rgroup behaves equivalent to a sibling task.
> * rgroup subtrees are local to the process. When the process forks or
> execs, its rgroup subtrees are collapsed.
> * When a process is migrated to a different cgroup, its rgroup
> subtrees are preserved.
> * Subset of controllers available on the parent sgroup are available
> to rgroup subtrees. Controller management on rgroups is automatic
> and implicit and doesn't interfere with system-level cgroup
> controller management. If a controller is made unavailable on the
> parent sgroup, it's automatically disabled from child rgroup
> subtrees.
> rgroup lays the foundation for other kernel mechanisms to make use of
> resource controllers while providing proper isolation between system
> management and in-process operations removing the awkward and
> layer-violating requirement for coordination between individual
> applications and system management. On top of the rgroup mechanism,
> PRIO_RGRP is implemented for {set|get}priority(2).
> * PRIO_RGRP can only be used if the target task is already in a
> rgroup. If setpriority(2) is used and cpu controller is available,
> cpu controller is enabled until the target rgroup is covered and the
> specified nice value is set as the weight of the rgroup.
> * The specified nice value has the same meaning as for tasks. For
> example, a rgroup and a task competing under the same parent would
> behave exactly the same as two tasks.
> * For top-level rgroups, PRIO_RGRP follows the same rlimit
> restrictions as PRIO_PROCESS; however, as nested rgroups only
> distribute CPU cycles which are allocated to the process, no
> restriction is applied.
> PRIO_RGRP allows in-process hierarchical control of CPU cycles in a
> manner which is a straight-forward and minimal extension of existing
> task and priority management.

Hrm. You're showing that per-thread groups can coexist just fine,
which is good given need and usage exists today out in the wild. Why
do such groups have to be invisible with a unique interface though?

Given the core has to deal with them whether they're visible or not,
and given they exist to fulfill a need, seems they should be first
class citizens, not some Quasimodo like creature sneaking into the
cathedral via a back door and slinking about in the shadows.