Re: [PATCH v5 00/10] track CPU utilization

Peter Zijlstra
2018-06-04T16:51:01Z

On Fri, May 25, 2018 at 03:12:21PM +0200, Vincent Guittot wrote:
> When both cfs and rt tasks compete to run on a CPU, we can see some frequency
> drops with schedutil governor. In such case, the cfs_rq's utilization doesn't
> reflect anymore the utilization of cfs tasks but only the remaining part that
> is not used by rt tasks. We should monitor the stolen utilization and take
> it into account when selecting OPP. This patchset doesn't change the OPP
> selection policy for RT tasks but only for CFS tasks

So the problem is that when RT/DL/stop/IRQ happens and preempts CFS
tasks, time continues and the CFS load tracking will see !running and
decay things.

Then, when we get back to CFS, we'll have lower load/util than we
expected.

In particular, your focus is on OPP selection, and where we would have
say: u=1 (always running task), after being preempted by our RT task for
a while, it will now have u=.5. With the effect that when the RT task
goes sleep we'll drop our OPP to .5 max -- which is 'wrong', right?

Your solution is to track RT/DL/stop/IRQ with the identical PELT average
as we track cfs util. Such that we can then add the various averages to
reconstruct the actual utilisation signal.

This should work for the case of the utilization signal on UP. When we
consider that PELT migrates the signal around on SMP, but we don't do
that to the per-rq signals we have for RT/DL/stop/IRQ.

There is also the 'complaint' that this ends up with 2 util signals for
DL, complicating things.

So this patch-set tracks the !cfs occupation using the same function,
which is all good. But what, if instead of using that to compensate the
OPP selection, we employ that to renormalize the util signal?

If we normalize util against the dynamic (rt_avg affected) cpu_capacity,
then I think your initial problem goes away. Because while the RT task
will push the util to .5, it will at the same time push the CPU capacity
to .5, and renormalized that gives 1.

NOTE: the renorm would then become something like:
scale_cpu = arch_scale_cpu_capacity() / rt_frac();

On IRC I mentioned stopping the CFS clock when preempted, and while that
would result in fixed numbers, Vincent was right in pointing out the
numbers will be difficult to interpret, since the meaning will be purely
CPU local and I'm not sure you can actually fix it again with
normalization.

Imagine, running a .3 RT task, that would push the (always running) CFS
down to .7, but because we discard all !cfs time, it actually has 1. If
we try and normalize that we'll end up with ~1.43, which is of course
completely broken.

_However_, all that happens for util, also happens for load. So the above
scenario will also make the CPU appear less loaded than it actually is.

Now, we actually try and compensate for that by decreasing the capacity
of the CPU. But because the existing rt_avg and PELT signals are so
out-of-tune, this is likely to be less than ideal. With that fixed
however, the best this appears to do is, as per the above, preserve the
actual load. But what we really wanted is to actually inflate the load,
such that someone will take load from us -- we're doing less actual work
after all.

Possibly, we can do something like:

scale_cpu_capacity / (rt_frac^2)

for load, then we inflate the load and could maybe get rid of all this
capacity_of() sprinkling, but that needs more thinking.

But I really feel we need to consider both util and load, as this issue
affects both.