Re: [PATCH v5 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp

[Dietmar Eggemann]

On 10/11/2020 13:21, Peter Zijlstra wrote:
> On Tue, Nov 03, 2020 at 10:37:56AM +0800, 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.
>>
>> The policy is
>> _CLAMP_RESET                           => reset both min and max
>> _CLAMP_RESET | _CLAMP_MIN              => reset min value
>> _CLAMP_RESET | _CLAMP_MAX              => reset max value
>> _CLAMP_RESET | _CLAMP_MIN | _CLAMP_MAX => reset both min and max
>>
> 
> The obvious alternative would be to use a magic value in
> sched_util_{min,max} to indicate reset. After all, we strictly enforce
> the values are inside [0,1024], which leaves us with many unused values.
> 
> Specifically -1 comes to mind. It would allow doing this without an
> extra flag, OTOH the explicit flag is well, more explicit.
> 
> I don't have a strong preference either way, but I wanted to make sure
> it was considered, and perhaps we can record why this isn't as nice a
> solution, dunno.

IMHO the '-1'  magic value approach is cleaner. Did some light testing on it.