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SubjectRe: [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.

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