On Tue, Sep 16, 2014 at 12:14:54AM +0200, Vincent Guittot wrote:

On 15 September 2014 13:42, Peter Zijlstra [off-list ref] wrote:

quoted

OK, I've reconsidered _again_, I still don't get it.

So fundamentally I think its wrong to scale with the capacity; it just
doesn't make any sense. Consider big.little stuff, their CPUs are
inherently asymmetric in capacity, but that doesn't matter one whit for
utilization numbers. If a core is fully consumed its fully consumed, no
matter how much work it can or can not do.


So the only thing that needs correcting is the fact that these
statistics are based on clock_task and some of that time can end up in
other scheduling classes, at which point we'll never get 100% even
though we're 'saturated'. But correcting for that using capacity doesn't
'work'.

I'm not sure to catch your last point because the capacity is the only
figures that take into account the "time" consumed by other classes.
Have you got in mind another way to take into account the other
classes ?

So that was the entire point of stuffing capacity in? Note that that
point was not at all clear.

This is very much like 'all we have is a hammer, and therefore
everything is a nail'. The rt fraction is a 'small' part of what the
capacity is.

So we have cpu_capacity that is the capacity that can be currently
used by cfs class
We have cfs.usage_load_avg that is the sum of running time of cfs
tasks on the CPU and reflect the % of usage of this CPU by CFS tasks
We have to use the same metrics to compare available capacity for CFS
and current cfs usage

-ENOPARSE

Now we have to use the same unit so we can either weight the
cpu_capacity_orig with the cfs.usage_load_avg and compare it with
cpu_capacity
or with divide cpu_capacity by cpu_capacity_orig and scale it into the
SCHED_LOAD_SCALE range. Is It what you are proposing ?

I'm so not getting it; orig vs capacity still includes
arch_scale_freq_capacity(), so that is not enough to isolate the rt
fraction.