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...

quoted

On Thu, Aug 22, 2019 at 02:28:06PM +0100, Patrick Bellasi wrote:

quoted

+#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.

quoted

quoted

+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.

quoted

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.