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

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Date:   Mon, 2 Sep 2019 09:47:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
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 Mon, Sep 02, 2019 at 07:38:53AM +0100, Patrick Bellasi wrote:
> 
> 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: 

Ooh, right you are. I clearly was in need of weekend. Somehow I read
that code as if you were forcing the float representation into an
integer, which is not what you do.

> 	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?

Yeah, let me go add that.

> >
> >> +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.

Right; I figured.

> > 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().

OK, I made it strcmp(), because that is what I figured was the intended
semantics.