Re: [PATCH 0/9] Enable haltpoll for arm64

[Ankur Arora <ankur.a.arora@oracle.com>]

> Subject: Re: [PATCH 0/9] Enable haltpoll for arm64

A correction: please read the subject for the series as [PATCH v5] ...

Missed the version number it while sending out.

Thanks
Ankur

Ankur Arora <ankur.a.arora@oracle.com> writes:

> This patchset enables the cpuidle-haltpoll driver and its namesake
> governor on arm64. This is specifically interesting for KVM guests by
> reducing the IPC latencies.
>
> Comparing idle switching latencies on an arm64 KVM guest with
> perf bench sched pipe:
>
>                                      usecs/op       %stdev
>
>   no haltpoll (baseline)               13.48       +-  5.19%
>   with haltpoll                         6.84       +- 22.07%
>
>
> No change in performance for a similar test on x86:
>
>                                      usecs/op        %stdev
>
>   haltpoll w/ cpu_relax() (baseline)     4.75      +-  1.76%
>   haltpoll w/ smp_cond_load_relaxed()    4.78      +-  2.31%
>
> Both sets of tests were on otherwise idle systems with guest VCPUs
> pinned to specific PCPUs. One reason for the higher stdev on arm64
> is that trapping of the WFE instruction by the host KVM is contingent
> on the number of tasks on the runqueue.
>
>
> The patch series is organized in four parts:
>  - patches 1, 2 mangle the config option ARCH_HAS_CPU_RELAX, renaming
>    and moving it from x86 to common architectural code.
>  - next, patches 3-5, reorganize the haltpoll selection and init logic
>    to allow architecture code to select it.
>  - patch 6, reorganizes the poll_idle() loop, switching from using
>    cpu_relax() directly to smp_cond_load_relaxed().
>  - and finally, patches 7-9, add the bits for arm64 support.
>
> What is still missing: this series largely completes the haltpoll side
> of functionality for arm64. There are, however, a few related areas
> that still need to be threshed out:
>
>  - WFET support: WFE on arm64 does not guarantee that poll_idle()
>    would terminate in halt_poll_ns. Using WFET would address this.
>  - KVM_NO_POLL support on arm64
>  - KVM TWED support on arm64: allow the host to limit time spent in
>    WFE.
>
>
> Changelog:
>
> v5:
>  - rework the poll_idle() loop around smp_cond_load_relaxed() (review
>    comment from Tomohiro Misono.)
>  - also rework selection of cpuidle-haltpoll. Now selected based
>    on the architectural selection of ARCH_CPUIDLE_HALTPOLL.
>  - arch_haltpoll_supported() (renamed from arch_haltpoll_want()) on
>    arm64 now depends on the event-stream being enabled.
>  - limit POLL_IDLE_RELAX_COUNT on arm64 (review comment from Haris Okanovic)
>  - ARCH_HAS_CPU_RELAX is now renamed to ARCH_HAS_OPTIMIZED_POLL.
>
> v4 changes from v3:
>  - change 7/8 per Rafael input: drop the parens and use ret for the final check
>  - add 8/8 which renames the guard for building poll_state
>
> v3 changes from v2:
>  - fix 1/7 per Petr Mladek - remove ARCH_HAS_CPU_RELAX from arch/x86/Kconfig
>  - add Ack-by from Rafael Wysocki on 2/7
>
> v2 changes from v1:
>  - added patch 7 where we change cpu_relax with smp_cond_load_relaxed per PeterZ
>    (this improves by 50% at least the CPU cycles consumed in the tests above:
>    10,716,881,137 now vs 14,503,014,257 before)
>  - removed the ifdef from patch 1 per RafaelW
>
> Ankur Arora (4):
>   cpuidle: rename ARCH_HAS_CPU_RELAX to ARCH_HAS_OPTIMIZED_POLL
>   cpuidle-haltpoll: condition on ARCH_CPUIDLE_HALTPOLL
>   arm64: support cpuidle-haltpoll
>   cpuidle/poll_state: limit POLL_IDLE_RELAX_COUNT on arm64
>
> Joao Martins (4):
>   Kconfig: move ARCH_HAS_OPTIMIZED_POLL to arch/Kconfig
>   cpuidle-haltpoll: define arch_haltpoll_supported()
>   governors/haltpoll: drop kvm_para_available() check
>   arm64: define TIF_POLLING_NRFLAG
>
> Mihai Carabas (1):
>   cpuidle/poll_state: poll via smp_cond_load_relaxed()
>
>  arch/Kconfig                              |  3 +++
>  arch/arm64/Kconfig                        | 10 ++++++++++
>  arch/arm64/include/asm/cpuidle_haltpoll.h | 21 +++++++++++++++++++++
>  arch/arm64/include/asm/thread_info.h      |  2 ++
>  arch/x86/Kconfig                          |  4 +---
>  arch/x86/include/asm/cpuidle_haltpoll.h   |  1 +
>  arch/x86/kernel/kvm.c                     | 10 ++++++++++
>  drivers/acpi/processor_idle.c             |  4 ++--
>  drivers/cpuidle/Kconfig                   |  5 ++---
>  drivers/cpuidle/Makefile                  |  2 +-
>  drivers/cpuidle/cpuidle-haltpoll.c        |  9 ++-------
>  drivers/cpuidle/governors/haltpoll.c      |  6 +-----
>  drivers/cpuidle/poll_state.c              | 21 ++++++++++++++++-----
>  drivers/idle/Kconfig                      |  1 +
>  include/linux/cpuidle.h                   |  2 +-
>  include/linux/cpuidle_haltpoll.h          |  5 +++++
>  16 files changed, 79 insertions(+), 27 deletions(-)
>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h


--
ankur