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

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Date:   Wed, 28 Oct 2020 18:29:46 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Patrick Bellasi <patrick.bellasi@...bug.net>
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 10/28/20 19:03, Patrick Bellasi wrote:
> 
> 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().

Updating the default rt value is done from different contexts. Hence it is
important to document the rules under which this update must happen and ensure
the update happens through a common path.

__setscheduler_uclamp() is not called from 2 different contexts.

> 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 disagree. This is unnecessary churn.

Thanks

--
Qais Yousef

> > 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.