Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks

From: Peter Zijlstra
Date: Thu Jan 24 2019 - 10:31:14 EST

On Thu, Jan 24, 2019 at 12:30:09PM +0000, Patrick Bellasi wrote:
> > So I'll have to go over the code again, but I'm wondering why you're
> > changing uclamp_se::bucket_id on a runnable task.
> We change only the "requested" value, not the "effective" one.
> > Ideally you keep bucket_id invariant between enqueue and dequeue; then
> > dequeue knows where we put it.
> Right, that's what we do for the "effective" value.

So the problem I have is that you first introduce uclamp_se::value and
use that all over the code, and then introduce effective and change all
the usage sites.

That seems daft. Why not keep all the code as-is and add orig_value.

> > Now I suppose actually determining bucket_id is 'expensive' (it
> > certainly is with the whole mapping scheme, but even that integer
> > division is not nice), so we'd like to precompute the bucket_id.
> Yes, although the complexity is mostly in the composition logic
> described above not on mapping at all. We have "mapping" overheads
> only when we change a "request" value and that's from slow-paths.

It's weird though. Esp. when combined with that mapping logic, because
then you get to use additional maps that are not in fact ever used.

> > We can update uclamp_se::value and set uclamp_se::changed, and then the
> > next enqueue will (unlikely) test-and-clear changed and recompute the
> > bucket_id.
> This mean will lazy update the "requested" bucket_id by deferring its
> computation at enqueue time. Which saves us a copy of the bucket_id,
> i.e. we will have only the "effective" value updated at enqueue time.
> But...
> > Would that not be simpler?
> ... although being simpler it does not fully exploit the slow-path,
> a syscall which is usually running from a different process context
> (system management software).
> It also fits better for lazy updates but, in the cgroup case, where we
> wanna enforce an update ASAP for RUNNABLE tasks, we will still have to
> do the updates from the slow-path.
> Will look better into this simplification while working on v7, perhaps
> the linear mapping can really help in that too.

OK. So mostly my complaint is that it seems to do things odd for ill
explained reasons.