Re: [PATCH v3 09/14] sched/core: uclamp: propagate parent clamps

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 17-Aug 15:43, Dietmar Eggemann wrote:
> On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> >In order to properly support hierarchical resources control, the cgroup
> >delegation model requires that attribute writes from a child group never
> >fail but still are (potentially) constrained based on parent's assigned
> >resources. This requires to properly propagate and aggregate parent
> >attributes down to its descendants.
> 
> I don't understand the reason mentioned here:
> 
> IMHO, a write to a child's (tg1/tg11) cpu.rt_runtime_us can fail if the
> value is restricted by the parents value:

Well... that's my interpretation after this discussion:

  https://lore.kernel.org/lkml/20180410200514.GA793541@devbig577.frc2.facebook.com/

AFAIU, what has not to fail is a write to a parent, which wants to enforce
more restrictive constraints to child groups. Thus, if we have for example:

   tg1:         util_max=100%
   tg1/tg11:    util_max=80%

It should be possible without errors to set:

   tg1:         util_max=50%

and then enforce a 50% util_max to tg1/tg11 tasks too and eventually
use "effective" attributes to expose the effective value used at each
level of the hierarchy.

> root@juno:/sys/fs/cgroup/cpu# cat cpu.rt_*
> 1000000
> 950000
> root@juno:/sys/fs/cgroup/cpu# cat tg1/cpu.rt_*
> 1000000
> 0
> root@juno:/sys/fs/cgroup/cpu# cat tg1/tg11/cpu.rt_*
> 1000000
> 0
> root@juno:/sys/fs/cgroup/cpu# echo 950000 > tg1/tg11/cpu.rt_runtime_us
> -bash: echo: write error: Invalid argument
> root@juno:/sys/fs/cgroup/cpu# echo 950000 > tg1/cpu.rt_runtime_us
> root@juno:/sys/fs/cgroup/cpu# echo 950000 > tg1/tg11/cpu.rt_runtime_us
> root@juno:/sys/fs/cgroup/cpu#

This example is using the legacy hierarcy (cgroups v1).

AFAIK the default hierarcy (cgroups v2) has a much more stricy set of
requirements for the "delegation model".

> >Let's implement this mechanism by adding a new "effective" clamp value
> >for each task group. The effective clamp value is defined as the smaller
> >value between the clamp value of a group and the effective clamp value
> >of its parent. This represent also the clamp value which is actually
> >used to clamp tasks in each task group.
> >
> >Since it can be interesting for tasks in a cgroup to know exactly what
> >is the currently propagated/enforced configuration, the effective clamp
> >values are exposed to user-space by means of a new pair of read-only
> >attributes: cpu.util.{min,max}.effective.
> 
> I assume here that the cpu.util.{min,max} of the child will not be used any
> more because the 'effective' counterparts are taken instead.

Yes, the "effective" attributes are the one used in kernel space for
the actual clamping.

However, the cpu.util.{min,max} of a child are still required as soon
as the parent relax its constraints... when we use their value to
set the "effective" value.

> I wonder if this propagation not been provided with only cpu.util.{min,max}?

In the example before, if we use the same variables we miss the
opportunity to reset:

   tg1/tg11:    util_max=80%

as soon as tg1's util_max goes back to 100%.

-- 
#include <best/regards.h>

Patrick Bellasi