linux-kernel - Re: [PATCH v5 2/2] sched/uclamp: Protect uclamp fast path code with static key

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Message-ID: <87wo3omot7.derkling@matbug.net>
Date:   Tue, 30 Jun 2020 19:44:04 +0200
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Chris Redpath <chris.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] sched/uclamp: Protect uclamp fast path code with static key


On Tue, Jun 30, 2020 at 17:40:34 +0200, Qais Yousef <qais.yousef@....com> wrote...

> Hi Patrick
>
> On 06/30/20 16:55, Patrick Bellasi wrote:
>> 
>> Hi Qais,
>> sorry for commenting on v5 with a v6 already posted, but...
>> ... I cannot keep up with your re-spinning rate ;)
>
> I classified that as a nit really and doesn't affect correctness. We have
> different subjective view on what is better here. I did all the work in the
> past 2 weeks and I think as the author of this patch I have the right to keep
> my preference on subjective matters. I did consider your feedback and didn't
> ignore it and improved the naming and added a comment to make sure there's no
> confusion.
>
> We could nitpick the best name forever, but is it really that important?

Which leans toward confirming the impression I had while reading your
previous response, i.e. you stopped reading at the name change
observation, which would be _just_ a nit-picking, although still worth
IMHO.

Instead, I went further and asked you to consider a different approach:
not adding a new kernel symbol to represent a concept already there.

> I really don't see any added value for one approach or another here to start
> a long debate about it.

Then you could have just called out that instead of silently ignoring
the comment/proposal.

> The comments were small enough that I didn't see any controversy that
> warrants holding the patches longer. I agreed with your proposal to use
> uc_se->active and clarified why your other suggestions don't hold.
>
> You pointed that uclamp_is_enabled() confused you; and I responded that I'll
> change the name.

Perhaps it would not confuse only me having 'something_enabled()'
referring to 'something_used'.

> Sorry for not being explicit about answering the below, but
> I thought my answer implied that I don't prefer it.

Your answer was about a name change, don't see correlation with a
different approach... but should be just me.

>> >> Thus, perhaps we can just use the same pattern used by the
>> >> sched_numa_balancing static key:
>> >> 
>> >>   $ git grep sched_numa_balancing
>> >>   kernel/sched/core.c:DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
>> >>   kernel/sched/core.c:            static_branch_enable(&sched_numa_balancing);
>> >>   kernel/sched/core.c:            static_branch_disable(&sched_numa_balancing);
>> >>   kernel/sched/core.c:    int state = static_branch_likely(&sched_numa_balancing);
>> >>   kernel/sched/fair.c:    if (!static_branch_likely(&sched_numa_balancing))
>> >>   kernel/sched/fair.c:    if (!static_branch_likely(&sched_numa_balancing))
>> >>   kernel/sched/fair.c:    if (!static_branch_likely(&sched_numa_balancing))
>> >>   kernel/sched/fair.c:    if (static_branch_unlikely(&sched_numa_balancing))
>> >>   kernel/sched/sched.h:extern struct static_key_false sched_numa_balancing;
>> >> 
>> >> IOW: unconditionally define sched_uclamp_used as non static in core.c,
>> >> and use it directly on schedutil too.
>> 
>> So, what about this instead of adding the (renamed) method above?
>
> I am sorry there's no written rule that says one should do it in a specific
> way. And AFAIK both way are implemented in the kernel. I appreciate your
> suggestion but as the person who did all the hard work, I think my preference
> matters here too.

You sure know that sometime reviewing code can be an "hard work" too, so I
would not go down that way at all with the discussion. Quite likely I
have a different "subjective" view on how Open Source development works.

> And actually with my approach when uclamp is not compiled in there's no need to
> define an extra variable; and since uclamp_is_used() is defined as false for
> !CONFIG_UCLAMP_TASK, it'll help with DCE, so less likely to end up with dead
> code that'll never run in the final binary.

Good, this is the simple and small reply I've politely asked for.

Best,
Patrick