Re: [PATCH 00/28] PM: QoS: Get rid of unuseful code and rework CPU latency QoS interface

[Rafael J. Wysocki]

On Thu, Feb 13, 2020 at 1:16 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Thu, Feb 13, 2020 at 12:31 AM Francisco Jerez <currojerez@xxxxxxxxxx> wrote:
> >
> > "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> writes:
> >
> > > Hi All,
> > >
> > > This series of patches is based on the observation that after commit
> > > c3082a674f46 ("PM: QoS: Get rid of unused flags") the only global PM QoS class
> > > in use is PM_QOS_CPU_DMA_LATENCY, but there is still a significant amount of
> > > code dedicated to the handling of global PM QoS classes in general.  That code
> > > takes up space and adds overhead in vain, so it is better to get rid of it.
> > >
> > > Moreover, with that unuseful code removed, the interface for adding QoS
> > > requests for CPU latency becomes inelegant and confusing, so it is better to
> > > clean it up.
> > >
> > > Patches [01/28-12/28] do the first part described above, which also includes
> > > some assorted cleanups of the core PM QoS code that doesn't go away.
> > >
> > > Patches [13/28-25/28] rework the CPU latency QoS interface (in the classic
> > > "define stubs, migrate users, change the API proper" manner), patches
> > > [26-27/28] update the general comments and documentation to match the code
> > > after the previous changes and the last one makes the CPU latency QoS depend
> > > on CPU_IDLE (because cpuidle is the only user of its target value today).
> > >
> > > The majority of the patches in this series don't change the functionality of
> > > the code at all (at least not intentionally).
> > >
> > > Please refer to the changelogs of individual patches for details.
> > >
> > > Thanks!
> >
> > Hi Rafael,
> >
> > I believe some of the interfaces removed here could be useful in the
> > near future.
>
> I disagree.
>
> >  It goes back to the energy efficiency- (and IGP graphics
> > performance-)improving series I submitted a while ago [1].  It relies on
> > some mechanism for the graphics driver to report an I/O bottleneck to
> > CPUFREQ, allowing it to make a more conservative trade-off between
> > energy efficiency and latency, which can greatly reduce the CPU package
> > energy usage of IO-bound applications (in some graphics benchmarks I've
> > seen it reduced by over 40% on my ICL laptop), and therefore also allows
> > TDP-bound applications to obtain a reciprocal improvement in throughput.
> >
> > I'm not particularly fond of the global PM QoS interfaces TBH, it seems
> > like an excessively blunt hammer to me, so I can very much relate to the
> > purpose of this series.  However the finer-grained solution I've
> > implemented has seen some push-back from i915 and CPUFREQ devs due to
> > its complexity, since it relies on task scheduler changes in order to
> > track IO bottlenecks per-process (roughly as suggested by Peter Zijlstra
> > during our previous discussions), pretty much in the spirit of PELT but
> > applied to IO utilization.
> >
> > With that in mind I was hoping we could take advantage of PM QoS as a
> > temporary solution [2], by introducing a global PM QoS class similar but
> > with roughly converse semantics to PM_QOS_CPU_DMA_LATENCY, allowing
> > device drivers to report a *lower* bound on CPU latency beyond which PM
> > shall not bother to reduce latency if doing so would have negative
> > consequences on the energy efficiency and/or parallelism of the system.
>
> So I really don't quite see how that could be responded to, by cpuidle
> say.  What exactly do you mean by "reducing latency" in particular?
>
> > Of course one would expect the current PM_QOS_CPU_DMA_LATENCY upper
> > bound to take precedence over the new lower bound in cases where the
> > former is in conflict with the latter.
>
> So that needs to be done on top of this series.
>
> > I can think of several alternatives to that which don't involve
> > temporarily holding off your clean-up,
>
> The cleanup goes in.  Please work on top of it.
>
> > but none of them sound particularly exciting:
> >
> >  1/ Use an interface specific to CPUFREQ, pretty much like the one
> >     introduced in my original submission [1].
>
> It uses frequency QoS already today, do you really need something else?
>
> >  2/ Use per-CPU PM QoS, which AFAICT would require the graphics driver
> >     to either place a request on every CPU of the system (which would
> >     cause a frequent operation to have O(N) complexity on the number of
> >     CPUs on the system), or play a cat-and-mouse game with the task
> >     scheduler.
>
> That's in place already too in the form of device PM QoS; see
> drivers/base/power/qos.c.
>
> >  3/ Add a new global PM QoS mechanism roughly duplicating the
> >     cpu_latency_qos_* interfaces introduced in this series.  Drop your
> >     change making this available to CPU IDLE only.
>
> It sounds like you really want performance for energy efficiency and
> CPU latency has a little to do with that.
>
> >  3/ Go straight to a scheduling-based approach, which is likely to
> >     greatly increase the review effort required to upstream this
> >     feature.  (Peter might disagree though?)
>
> Are you familiar with the utilization clamps mechanism?

And BTW, posting patches as RFC is fine even if they have not been
tested.  At least you let people know that you work on something this
way, so if they work on changes in the same area, they may take that
into consideration.

Also if there are objections to your proposal, you may save quite a
bit of time by sending it early.

It is unfortunate that this series has clashed with the changes that
you were about to propose, but in this particular case in my view it
is better to clean up things and start over.

Thanks!