Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

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

On Thu, Sep 05, 2019 at 09:31:27 +0100, Peter Zijlstra wrote...

> On Fri, Aug 30, 2019 at 10:49:36AM -0700, subhra mazumdar wrote:
>> Add Cgroup interface for latency-nice. Each CPU Cgroup adds a new file
>> "latency-nice" which is shared by all the threads in that Cgroup.
>
> *sigh*, no. We start with a normal per task attribute, and then later,
> if it is needed and makes sense, we add it to cgroups.

FWIW, to add on top of what Peter says, we used this same approach for
uclamp and it proved to be a very effective way to come up with a good
design. General principles have been:

 - a system wide API [1] (under /proc/sys/kernel/sched_*) defines
   default values for all tasks affected by that feature.
   This interface has to define also upper bounds for task specific
   values. Thus, in the case of latency-nice, it should be set by
   default to the MIN value, since that's the current mainline
   behaviour: all tasks are latency sensitive.

 - a per-task API [2] (via the sched_setattr() syscall) can be used to
   relax the system wide setting thus implementing a "nice" policy.

 - a per-taskgroup API [3] (via cpu controller's attributes) can be used
   to relax the system-wide settings and restrict the per-task API.

The above features are worth to be added in that exact order.

> Also, your Changelog fails on pretty much every point. It doesn't
> explain why, it doesn't describe anything and so on.

On the description side, I guess it's worth to mention somewhere to
which scheduling classes this feature can be useful for. It's worth to
mention that it can apply only to:

 - CFS tasks: for example, at wakeup time a task with an high
   latency-nice should avoid to preempt a low latency-nice task.
   Maybe by mapping the latency nice value into proper vruntime
   normalization value?

 - RT tasks: for example, at wakeup time a task with an high
   latency-nice value could avoid to preempt a CFS task.

I'm sure there will be discussion about some of these features, that's
why it's important in the proposal presentation to keep a well defined
distinction among the "mechanisms and API" and how we use the new
concept to "bias" some scheduler policies.

> From just reading the above, I would expect it to have the range
> [-20,19] just like normal nice. Apparently this is not so.

Regarding the range for the latency-nice values, I guess we have two
options:

  - [-20..19], which makes it similar to priorities
  downside: we quite likely end up with a kernel space representation
  which does not match the user-space one, e.g. look at
  task_struct::prio.

  - [0..1024], which makes it more similar to a "percentage"

Being latency-nice a new concept, we are not constrained by POSIX and
IMHO the [0..1024] scale is a better fit.

That will translate into:

  latency-nice=0 : default (current mainline) behaviour, all "biasing"
  policies are disabled and we wakeup up as fast as possible

  latency-nice=1024 : maximum niceness, where for example we can imaging
  to turn switch a CFS task to be SCHED_IDLE?

Best,
Patrick

[1] commit e8f14172c6b1 ("sched/uclamp: Add system default clamps")
[2] commit a509a7cd7974 ("sched/uclamp: Extend sched_setattr() to support utilization clamping")
[3] 5 patches in today's tip/sched/core up to:
    commit babbe170e053 ("sched/uclamp: Update CPU's refcount on TG's clamp changes")

-- 
#include <best/regards.h>

Patrick Bellasi