Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

Patrick Bellasi
2019-03-14T12:13:15Z

On 13-Mar 20:48, Peter Zijlstra wrote:
> On Wed, Mar 13, 2019 at 04:12:29PM +0000, Patrick Bellasi wrote:
> > On 13-Mar 14:40, Peter Zijlstra wrote:
> > > On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
> > > > +static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
> > > > +{
> > > > + return clamp_value / UCLAMP_BUCKET_DELTA;
> > > > +}
> > > > +
> > > > +static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
> > > > +{
> > > > + return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);
> > >
> > > return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA);
> > >
> > > might generate better code; just a single division, instead of a div and
> > > mult.
> >
> > Wondering if compilers cannot do these optimizations... but yes, looks
> > cool and will do it in v8, thanks.
>
> I'd be most impressed if they pull this off. Check the generated code
> and see I suppose :-)

On x86 the code generated looks exactly the same:

https://godbolt.org/z/PjmA7k

While on on arm64 it seems the difference boils down to:
- one single "mul" instruction
vs
- two instructions: "sub" _plus_ one "multiply subtract"

https://godbolt.org/z/0shU0S

So, if I din't get something wrong... perhaps the original version is
even better, isn't it?

Test code:

---8<---
#define UCLAMP_BUCKET_DELTA 52

static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
{
return clamp_value / UCLAMP_BUCKET_DELTA;
}

static inline unsigned int uclamp_bucket_value1(unsigned int clamp_value)
{
return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);
}

static inline unsigned int uclamp_bucket_value2(unsigned int clamp_value)
{
return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA);
}

int test1(int argc, char *argv[]) {
return uclamp_bucket_value1(argc);
}

int test2(int argc, char *argv[]) {
return uclamp_bucket_value2(argc);
}

int test3(int argc, char *argv[]) {
return uclamp_bucket_value1(argc) - uclamp_bucket_value2(argc);
}
---8<---

which gives on arm64:

---8<---
test1:
mov w1, 60495
movk w1, 0x4ec4, lsl 16
umull x0, w0, w1
lsr x0, x0, 36
mov w1, 52
mul w0, w0, w1
ret
test2:
mov w1, 60495
movk w1, 0x4ec4, lsl 16
umull x1, w0, w1
lsr x1, x1, 36
mov w2, 52
msub w1, w1, w2, w0
sub w0, w0, w1
ret
test3:
mov w0, 0
ret
---8<---

--
#include <best/regards.h>

Patrick Bellasi