Re: [PATCH v3 11/26] fs/resctrl: Add support for additional monitor event display formats

Luck, Tony
2025-04-21T19:34:33Z

On Fri, Apr 18, 2025 at 04:02:08PM -0700, Reinette Chatre wrote:
> > + case EVT_TYPE_U46_18:
> > + frac = val & FRAC_MASK;
> > + frac = frac * 1000000;
> > + frac += 1ul << (NUM_FRAC_BITS - 1);
>
> Could you please help me understand why above line is needed? Seems like
> shift below will just undo it?
>
> > + frac >>= NUM_FRAC_BITS;
> > + seq_printf(m, "%llu.%06llu\n", val >> NUM_FRAC_BITS, frac);
> > + break;
> > + }

The extra addtion is to round the value to nearest decimal supported
value.

E.g. take the case where val == 1 This is a 1 in the 18th binary
place, so the precise decimal representation is:

1 / 2^18 = 0.000003814697265625

rounding to six decimal places should give: 0.000004

If you run the above code without the small addition you get:

frac = val & FRAC_MASK; // frac == 1
frac = frac * 1000000; // frac == 1000000
frac >>= NUM_FRAC_BITS; // frac == 3

So the output with be the truncated, not rounded, 0.000003

The addition will have a "carry bit" in to the upper bits.
That isn't lost when shifting right. The value added is
as if there was a "1" in the 19th binary place.

-Tony