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

[Francisco Jerez]

"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.  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.
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.

I can think of several alternatives to that which don't involve
temporarily holding off your clean-up, 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].
 
 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.
 
 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.
 
 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?)

Regards,
Francisco.

[1] https://lore.kernel.org/linux-pm/20180328063845.4884-1-currojerez@xxxxxxxxxx/

[2] I've written the code to do this already, but I wasn't able to test
    and debug it extensively until this week due to the instability of
    i915 on recent v5.5 kernels that prevented any benchmark run from
    surviving more than a few hours on my ICL system, hopefully the
    required i915 fixes will flow back to stable branches soon enough.

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