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 16:55:42 +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


Hi Qais,
sorry for commenting on v5 with a v6 already posted, but...
... I cannot keep up with your re-spinning rate ;)

More importantly, perhaps you missed to comment on one of my previous
points.

Will have a better look at the rest of v6 later today.

Cheers,
Patrick

On Tue, Jun 30, 2020 at 11:46:24 +0200, Qais Yousef <qais.yousef@....com> wrote...
> On 06/30/20 10:11, Patrick Bellasi wrote:
>> On Mon, Jun 29, 2020 at 18:26:33 +0200, Qais Yousef <qais.yousef@....com> wrote...

[...]

>> > +
>> > +static inline bool uclamp_is_enabled(void)
>> > +{
>> > +	return static_branch_likely(&sched_uclamp_used);
>> > +}
>> 
>> Looks like here we mix up terms, which can be confusing.
>> AFAIKS, we use:
>> - *_enabled for the sched class flags (compile time)
>> - *_used    for the user-space opting in (run time)
>
> I wanted to add a comment here.
>
> I can rename it to uclamp_is_used() if you want.

In my previous message I was mostly asking about this:

>> 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?