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SubjectRe: [PATCH v2 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp
Hi Yun,

On 12/10/2020 18:31, Yun Hsiang wrote:
> If the user wants to stop controlling uclamp and let the task inherit
> the value from the group, we need a method to reset.
> Add SCHED_FLAG_UTIL_CLAMP_RESET flag to allow the user to reset uclamp via
> sched_setattr syscall.

before we decide on how to implement the 'uclamp user_defined reset'
feature, could we come back to your use case in ?

Lets just consider uclamp min for now. We have:

(1) system-wide:

# cat /proc/sys/kernel/sched_util_clamp_min


(2) tg (hierarchy) with top-app's cpu.uclamp.min to ~200 (20% of 1024):

# cat /sys/fs/cgroup/cpu/top-app/cpu.uclamp.min

(3) and 2 cfs tasks A and B in top-app:

# cat /sys/fs/cgroup/cpu/top-app/tasks


Then you set A and B's uclamp min to 100. A and B are now user_defined.
A and B's effective uclamp min value is 100.

Since the task uclamp min values (3) are less than (1) and (2), their
uclamp min value is not affected by (1) or (2).

If A doesn't want to control itself anymore, it can set its uclamp min
to e.g. 300. Now A's effective uclamp min value is ~200, i.e. controlled
by (2), the one of B stays 100.

So the policy is:

(a) If the user_defined task wants to control it's uclamp, use task
uclamp value less than the tg (hierarchy) (and the system-wide)

(b) If the user_defined task doesn't want to control it's uclamp
anymore, use a uclamp value greater than or equal the tg (hierarchy)
(and the system-wide) value.

So where exactly is the use case which would require a 'uclamp
user_defined reset' functionality?

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