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

[Morten Rasmussen]

On Wed, Sep 09, 2015 at 03:23:43PM -0700, bsegall@xxxxxxxxxx wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > On Tue, Sep 08, 2015 at 03:31:58PM +0100, Morten Rasmussen wrote:
> >> On Tue, Sep 08, 2015 at 02:52:05PM +0200, Peter Zijlstra wrote:
> >> > > Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10.
> >> 
> >> I don't get why LOAD_AVG_MAX relies on the util_avg shifting being
> >> 1<<10, it is just the sum of the geometric series and the upper bound of
> >> util_sum?
> >
> > It needs a 1024, it might just have been the 1024 ns we use a period
> > instead of the scale unit though.
> >
> > The LOAD_AVG_MAX is the number where adding a next element to the series
> > doesn't change the result anymore, so scaling it up will allow more
> > significant elements to the series before we bottom out, which is the _N
> > thing.
> >
> 
> Yes, as the comments say, the 1024ns unit is arbitrary (and is an
> average of not-quite-microseconds instead of just nanoseconds to allow
> more bits to load.weight when we multiply load.weight by this number).
> In fact there are two arbitrary 1024 units here, which are technically
> unrelated and are both unrelated to SCHED_LOAD_RESOLUTION/etc - we
> operate on units of almost-microseconds and we also do decays every
> almost-millisecond.
> 
> There appears to be a bunch of confusion in the current code around
> util_sum/util_avg which appears to using SCHED_LOAD_SCALE
> for a fixed-point percentage or something, which is at least reasonable,
> but is initializing it as scale_load_down(SCHED_LOAD_SCALE), which
> results in either initializing as 100% or .1% depending on RESOLUTION.
> This'll get clobbered on first update, but if it needs to be
> initialized, it should either get initialized to something sane or at
> least consistent.

This is what I thought too. The whole geometric series math is completely
independent of the scale used for priority in load_avg and the fixed
point shifting used for util_avg.

> load_sum/load_avg appear to be scale_load_down()ed properly, and appear
> to be used as such at a quick glance.

I don't think shifting by SCHED_LOAD_SHIFT in __update_load_avg() is
right:

	sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX;

util_avg is initialized to low resolution (>> SCHED_LOAD_RESOLUTION):

        sa->util_avg = scale_load_down(SCHED_LOAD_SCALE);

so it appear to be intended to be using low resolution like load_avg
(weight is scaled down before it is passed into __update_load_avg()),
but util_avg is shifted up to high resolution. It should be:

	sa->util_avg = (sa->util_sum << (SCHED_LOAD_SHIFT -
					SCHED_LOAD_SHIFT)) / LOAD_AVG_MAX;

to be consistent.
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