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

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Date:   Tue, 30 Jun 2020 16:40:34 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Patrick Bellasi <patrick.bellasi@...bug.net>
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

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?

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

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

> 
> >> 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.

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.

Thanks a lot for all of your comments and feedback anyway!

--
Qais Yousef