Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

[Yuyang Du]

On Tue, Sep 08, 2015 at 01:50:38PM +0100, Dietmar Eggemann wrote:
> > It's both a unit and a SCALE/SHIFT problem, SCHED_LOAD_SHIFT and
> > SCHED_CAPACITY_SHIFT are defined separately so we must be sure to
> > scale the value in the right range. In the case of cpu_usage which
> > returns sa->util_avg , it's the capacity range not the load range.
> 
> Still don't understand why it's a unit problem. IMHO LOAD/UTIL and
> CAPACITY have no unit.

To be more accurate, probably, LOAD can be thought of as having unit,
but UTIL has no unit.
 
Anyway, those are my definitions:

1) unit, only for LOAD, and SCHED_LOAD_X is the unit (but
   SCHED_LOAD_RESOLUTION make it also some 2, see below)
2) range, aka, resolution or fix-point percentage (as Ben said)
3) timing ratio, LOAD_AVG_MAX etc, unralated with SCHED_LOAD_X

> >> I always thought that scale_load_down() takes care of that.
> > 
> > AFAIU, scale_load_down is a way to increase the resolution  of the
> > load not to move from load to capacity
> 
> I tried to figure out why we have this issue when comparing UTIL w/
> CAPACITY and not LOAD w/ CAPACITY:
> 
> Both are initialized like that:
> 
>  sa->load_avg = scale_load_down(se->load.weight);
>  sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
>  sa->util_avg = scale_load_down(SCHED_LOAD_SCALE);
>  sa->util_sum = LOAD_AVG_MAX;
> 
> and we use 'se->on_rq * scale_load_down(se->load.weight)' as 'unsigned
> long weight' argument to call __update_load_avg() making sure the
> scaling differences between LOAD and CAPACITY are respected while
> updating sa->load_sum (and sa->load_avg).

Yes, because we used SCHED_LOAD_X as both unit and range for LOAD.
 
> OTAH, we don't apply a scale_load_down for sa->util_[sum/avg] only a '<<
> SCHED_LOAD_SHIFT) / LOAD_AVG_MAX' on sa->util_avg.
> So changing '<< SCHED_LOAD_SHIFT' to '*
> scale_load_down(SCHED_LOAD_SCALE)' would be the logical thing to do.

Actually, for UTIL, we only need range, so don't conflate with LOAD,
what about we get all these clarified by redefining SCHED_LOAD_RESOLUTION
as the resolution/range generic macro for LOAD, UTIL, and CAPACITY:

#define SCHED_RESOLUTION_SHIFT  10
#define SCHED_RESOLUTION_SCALE  (1L << SCHED_RESOLUTION_SHIFT)

#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power usage under light load  */
# define scale_load(w)          ((w) << SCHED_RESOLUTION_SHIFT)
# define scale_load_down(w)     ((w) >> SCHED_RESOLUTION_SHIFT)
# define SCHED_LOAD_SHIFT       (10 + SCHED_RESOLUTION_SHIFT)
#else
# define scale_load(w)          (w)
# define scale_load_down(w)     (w)
# define SCHED_LOAD_SHIFT       (10)
#endif

#define SCHED_LOAD_SCALE        (1L << SCHED_LOAD_SHIFT)

For UTIL, e.g., it will be initiated as:
sa->util_avg = SCHED_RESOLUTION_SCALE;

And for capacity, we just use SCHED_RESOLUTION_SHIFT
(so SCHED_CAPACITY_SHIFT is not needed).

Thanks,
Yuyang
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