Re: [PATCH v10 00/11] arm64: support poll_idle()

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

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

> Hi,
>
> This patchset adds support for polling in idle on arm64 via poll_idle()
> and adds the requisite support to acpi-idle and cpuidle-haltpoll.
>
> v10 is a respin of v9 with the timed wait barrier logic
> (smp_cond_load_relaxed_timewait()) moved out into a separate
> series [0]. (The barrier patches could also do with some eyes.)

Sent a v2 for the barrier series:
  https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/

Ankur

> Why poll in idle?
> ==
>
> The benefit of polling in idle is to reduce the cost (and latency)
> of remote wakeups. When enabled, these can be done just by setting the
> need-resched bit, eliding the IPI, and the cost of handling the
> interrupt on the receiver.
>
> Comparing sched-pipe performance on a guest VM:
>
> # perf stat -r 5 --cpu 4,5 -e task-clock,cycles,instructions \
>    -e sched:sched_wake_idle_without_ipi perf bench sched pipe -l 1000000 --cpu 4
>
> # without polling in idle
>
>  Performance counter stats for 'CPU(s) 4,5' (5 runs):
>
>          25,229.57 msec task-clock                       #    2.000 CPUs utilized               ( +-  7.75% )
>     45,821,250,284      cycles                           #    1.816 GHz                         ( +- 10.07% )
>     26,557,496,665      instructions                     #    0.58  insn per cycle              ( +-  0.21% )
>                  0      sched:sched_wake_idle_without_ipi #    0.000 /sec
>
>             12.615 +- 0.977 seconds time elapsed  ( +-  7.75% )
>
>
> # polling in idle (with haltpoll):
>
>  Performance counter stats for 'CPU(s) 4,5' (5 runs):
>
>          15,131.58 msec task-clock                       #    2.000 CPUs utilized               ( +- 10.00% )
>     34,158,188,839      cycles                           #    2.257 GHz                         ( +-  6.91% )
>     20,824,950,916      instructions                     #    0.61  insn per cycle              ( +-  0.09% )
>          1,983,822      sched:sched_wake_idle_without_ipi #  131.105 K/sec                      ( +-  0.78% )
>
>              7.566 +- 0.756 seconds time elapsed  ( +- 10.00% )
>
> Comparing the two cases, there's a significant drop in both cycles and
> instructions executed. And a signficant drop in the wakeup latency.
>
> Tomohiro Misono and Haris Okanovic also report similar latency
> improvements on Grace and Graviton systems (for v7) [1] [2].
> Haris also tested a modified v9 on top of the split out barrier
> primitives.
>
> Lifeng also reports improved context switch latency on a bare-metal
> machine with acpi-idle [3].
>
>
> Series layout
> ==
>
>  - patches 1-3,
>
>     "cpuidle/poll_state: poll via smp_cond_load_relaxed_timewait()"
>     "cpuidle: rename ARCH_HAS_CPU_RELAX to ARCH_HAS_OPTIMIZED_POLL"
>     "Kconfig: move ARCH_HAS_OPTIMIZED_POLL to arch/Kconfig"
>
>    switch poll_idle() to using the new barrier interface. Also, do some
>    munging of related kconfig options.
>
>  - patches 4-5,
>
>     "arm64: define TIF_POLLING_NRFLAG"
>     "arm64: add support for poll_idle()"
>
>    add arm64 support for the polling flag and enable poll_idle()
>    support.
>
>  - patches 6, 7-11,
>
>     "ACPI: processor_idle: Support polling state for LPI"
>
>     "cpuidle-haltpoll: define arch_haltpoll_want()"
>     "governors/haltpoll: drop kvm_para_available() check"
>     "cpuidle-haltpoll: condition on ARCH_CPUIDLE_HALTPOLL"
>
>     "arm64: idle: export arch_cpu_idle()"
>     "arm64: support cpuidle-haltpoll"
>
>     add support for polling via acpi-idle, and cpuidle-haltpoll.
>
>
> Changelog
> ==
>
> v10: respin of v9
>    - sent out smp_cond_load_relaxed_timeout() separately [0]
>      - Dropped from this series:
>         "asm-generic: add barrier smp_cond_load_relaxed_timeout()"
>         "arm64: barrier: add support for smp_cond_relaxed_timeout()"
>         "arm64/delay: move some constants out to a separate header"
>         "arm64: support WFET in smp_cond_relaxed_timeout()"
>
>    - reworded some commit messages
>
> v9:
>  - reworked the series to address a comment from Catalin Marinas
>    about how v8 was abusing semantics of smp_cond_load_relaxed().
>  - add poll_idle() support in acpi-idle (Lifeng Zheng)
>  - dropped some earlier "Tested-by", "Reviewed-by" due to the
>    above rework.
>
> v8: No logic changes. Largely respin of v7, with changes
> noted below:
>
>  - move selection of ARCH_HAS_OPTIMIZED_POLL on arm64 to its
>    own patch.
>    (patch-9 "arm64: select ARCH_HAS_OPTIMIZED_POLL")
>
>  - address comments simplifying arm64 support (Will Deacon)
>    (patch-11 "arm64: support cpuidle-haltpoll")
>
> v7: No significant logic changes. Mostly a respin of v6.
>
>  - minor cleanup in poll_idle() (Christoph Lameter)
>  - fixes conflicts due to code movement in arch/arm64/kernel/cpuidle.c
>    (Tomohiro Misono)
>
> v6:
>
>  - reordered the patches to keep poll_idle() and ARCH_HAS_OPTIMIZED_POLL
>    changes together (comment from Christoph Lameter)
>  - threshes out the commit messages a bit more (comments from Christoph
>    Lameter, Sudeep Holla)
>  - also rework selection of cpuidle-haltpoll. Now selected based
>    on the architectural selection of ARCH_CPUIDLE_HALTPOLL.
>  - moved back to arch_haltpoll_want() (comment from Joao Martins)
>    Also, arch_haltpoll_want() now takes the force parameter and is
>    now responsible for the complete selection (or not) of haltpoll.
>  - fixes the build breakage on i386
>  - fixes the cpuidle-haltpoll module breakage on arm64 (comment from
>    Tomohiro Misono, Haris Okanovic)
>
> 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
>
>
> Would appreciate any review comments.
>
> Ankur
>
>
> [0] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
> [1] https://lore.kernel.org/lkml/TY3PR01MB111481E9B0AF263ACC8EA5D4AE5BA2@TY3PR01MB11148.jpnprd01.prod.outlook.com/
> [2] https://lore.kernel.org/lkml/104d0ec31cb45477e27273e089402d4205ee4042.camel@amazon.com/
> [3] https://lore.kernel.org/lkml/f8a1f85b-c4bf-4c38-81bf-728f72a4f2fe@huawei.com/
>
> Ankur Arora (6):
>   cpuidle/poll_state: poll via smp_cond_load_relaxed_timewait()
>   cpuidle: rename ARCH_HAS_CPU_RELAX to ARCH_HAS_OPTIMIZED_POLL
>   arm64: add support for poll_idle()
>   cpuidle-haltpoll: condition on ARCH_CPUIDLE_HALTPOLL
>   arm64: idle: export arch_cpu_idle()
>   arm64: support cpuidle-haltpoll
>
> Joao Martins (4):
>   Kconfig: move ARCH_HAS_OPTIMIZED_POLL to arch/Kconfig
>   arm64: define TIF_POLLING_NRFLAG
>   cpuidle-haltpoll: define arch_haltpoll_want()
>   governors/haltpoll: drop kvm_para_available() check
>
> Lifeng Zheng (1):
>   ACPI: processor_idle: Support polling state for LPI
>
>  arch/Kconfig                              |  3 ++
>  arch/arm64/Kconfig                        |  7 ++++
>  arch/arm64/include/asm/cpuidle_haltpoll.h | 20 +++++++++++
>  arch/arm64/include/asm/thread_info.h      |  2 ++
>  arch/arm64/kernel/idle.c                  |  1 +
>  arch/x86/Kconfig                          |  5 ++-
>  arch/x86/include/asm/cpuidle_haltpoll.h   |  1 +
>  arch/x86/kernel/kvm.c                     | 13 +++++++
>  drivers/acpi/processor_idle.c             | 43 +++++++++++++++++++----
>  drivers/cpuidle/Kconfig                   |  5 ++-
>  drivers/cpuidle/Makefile                  |  2 +-
>  drivers/cpuidle/cpuidle-haltpoll.c        | 12 +------
>  drivers/cpuidle/governors/haltpoll.c      |  6 +---
>  drivers/cpuidle/poll_state.c              | 27 +++++---------
>  drivers/idle/Kconfig                      |  1 +
>  include/linux/cpuidle.h                   |  2 +-
>  include/linux/cpuidle_haltpoll.h          |  5 +++
>  17 files changed, 105 insertions(+), 50 deletions(-)
>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h


--
ankur