linux-kernel - Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller

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Message-ID: <87zhjnnqz2.fsf@arm.com>
Date:   Mon, 02 Sep 2019 07:38:53 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, cgroups@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>, Michal Koutny <mkoutny@...e.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alessio Balsini <balsini@...roid.com>
Subject: Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller

On Fri, Aug 30, 2019 at 09:45:05 +0000, Peter Zijlstra wrote...

> On Thu, Aug 22, 2019 at 02:28:06PM +0100, Patrick Bellasi wrote:
>> +#define _POW10(exp) ((unsigned int)1e##exp)
>> +#define POW10(exp) _POW10(exp)
>
> What is this magic? You're forcing a float literal into an integer.
> Surely that deserves a comment!

Yes, I'm introducing the two constants:
  UCLAMP_PERCENT_SHIFT,
  UCLAMP_PERCENT_SCALE
similar to what we have for CAPACITY. Moreover, I need both 100*100 (for
the scale) and 100 further down in the code for the: 

	percent = div_u64_rem(percent, POW10(UCLAMP_PERCENT_SHIFT), &rem);

used in cpu_uclamp_print().

That's why adding a compile time support to compute a 10^N is useful.

C provides the "1eN" literal, I just convert it to integer and to do
that at compile time I need a two level macros.

What if I add this comment just above the macro definitions:

/*
 * Integer 10^N with a given N exponent by casting to integer the literal "1eN"
 * C expression. Since there is no way to convert a macro argument (N) into a
 * character constant, use two levels of macros.
 */

is this clear enough?

>
>> +struct uclamp_request {
>> +#define UCLAMP_PERCENT_SHIFT	2
>> +#define UCLAMP_PERCENT_SCALE	(100 * POW10(UCLAMP_PERCENT_SHIFT))
>> +	s64 percent;
>> +	u64 util;
>> +	int ret;
>> +};
>> +
>> +static inline struct uclamp_request
>> +capacity_from_percent(char *buf)
>> +{
>> +	struct uclamp_request req = {
>> +		.percent = UCLAMP_PERCENT_SCALE,
>> +		.util = SCHED_CAPACITY_SCALE,
>> +		.ret = 0,
>> +	};
>> +
>> +	buf = strim(buf);
>> +	if (strncmp("max", buf, 4)) {
>
> That is either a bug, and you meant to write: strncmp(buf, "max", 3),
> or it is not, and then you could've written: strcmp(buf, "max")

I don't think it's a bug.

The usage of 4 is intentional, to force a '\0' check while using
strncmp(). Otherwise, strncmp(buf, "max", 3) would accept also strings
starting by "max", which we don't want.

> But as written it doesn't make sense.

The code is safe but I agree that strcmp() does just the same and it
does not generate confusion. That's actually a pretty good example
on how it's not always better to use strncmp() instead of strcmp().

Cheers,
Patrick