linux-kernel - Re: [PATCH v3 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP

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Message-ID: <87sg9ymdmw.derkling@matbug.net>
Date:   Wed, 28 Oct 2020 19:03:35 +0100
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Yun Hsiang <hsiang023167@...il.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET
 flag to reset uclamp


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