Re: [PATCH v3 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp

From: Patrick Bellasi
Date: Wed Oct 28 2020 - 17:54:12 EST



On Wed, Oct 28, 2020 at 12:39:43 +0100, Qais Yousef <qais.yousef@xxxxxxx> wrote...

> On 10/28/20 11:11, Patrick Bellasi wrote:
>> >>
>> >> /*
>> >> * RT by default have a 100% boost value that could be modified
>> >> * at runtime.
>> >> */
>> >> if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
>> >> - __uclamp_update_util_min_rt_default(p);
>> >> + value = sysctl_sched_uclamp_util_min_rt_default;
>>
>> By removing this usage of __uclamp_updadate_util_min_rt_default(p),
>> the only other usage remaining is the call from:
>> uclamp_udpate_util_min_rt_default().
>>
>> What about an additional cleanup by in-lining the only surviving usage?
>
> This is not a cleanup IMO. There is special rule about updating that are
> encoded and documented in this helper function. Namely:
>
> * p->pi_lock must be held.
> * p->uclamp_req[].user_defined must be false.

Both these conditions are satisfied in the above call site:
- user_defined is tested just 4 lines above
- pi_lock is taken by the caller, i.e. __sched_setscheduler()
Thus, there is no need to test them two times.

Moreover, the same granted pi_lock you check in
__ucalmp_update_util_min_rt_default() is not checked at all in the rest
of __setscheduler_uclamp().

Thus, perhaps we should have just avoided to add
__uclamp_update_util_min_rt_default() since the beginning and:
- have all its logic in the _only_ place where it's required
- added the lockdep_assert_held() in __setscheduler_uclamp()

That's why I consider this a very good cleanup opportunity.

> I don't see open coding helps but rather makes the code harder to read and
> prone to introduce bugs if anything gets reshuffled in the future.

It's not open coding IMHO, it's just adding the code that's required.