mbox series

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

Message ID 20250218213337.377987-1-ankur.a.arora@oracle.com
Headers show
Series arm64: support poll_idle() | expand

Message

Ankur Arora Feb. 18, 2025, 9:33 p.m. UTC 
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.)


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