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

Patrick Bellasi
2019-01-24T16:14:43Z

On 24-Jan 16:31, Peter Zijlstra wrote:
> 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.

Right, because the moment we introduce the combination/aggregation mechanism
is the moment "effective" value makes sense to have. That's when the
code show that:

a task cannot always get what it "request", an "effective" value is
computed by aggregation and that's the value we use now for actual
clamp enforcing.

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

If you prefer, I can use effective values since the beginning and then
add the "requested" values later... but I fear the patchset will not
be more clear to parse.

> > > 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.

Mmm... don't get this point...

AFAICS "mapping" and "effective" are two different concepts, that's
why I can probably get rid of the first by I would prefer to keep the
second.

> > > 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.

:( I'm really sorry I'm not able to convey the overall design idea.

TBH however, despite being a quite "limited" feature it has many
different viewpoints: task-specific, cgroups and system defaults.

I've really tried my best to come up with something reasonable but I
understand that, looking at the single patches, the overall design
could be difficult to grasp... without considering that optimizations
are always possible of course.

If you like better, I can try to give a respin by just removing the
mapping part and then we go back and see if the reaming bits makes
more sense ?

--
#include <best/regards.h>

Patrick Bellasi