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

On 9/5/19 3:15 PM, Patrick Bellasi wrote:
> 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?

If I got this correct, does this also mean that a task's latency-nice
will be mapped to prio/nice.
i.e, task with min-latency-nice will have highest priority?

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

So, will this make CFS task to precede RT task?
and cause priority inversion?

> 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")

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