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Message-ID: <87wmaljd81.fsf@oracle.com>
Date: Mon, 12 May 2025 22:23:26 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-pm@...r.kernel.org, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-acpi@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
x86@...nel.org, pbonzini@...hat.com, vkuznets@...hat.com,
rafael@...nel.org, daniel.lezcano@...aro.org, peterz@...radead.org,
arnd@...db.de, lenb@...nel.org, mark.rutland@....com,
harisokn@...zon.com, mtosatti@...hat.com, sudeep.holla@....com,
cl@...two.org, maz@...nel.org, misono.tomohiro@...itsu.com,
maobibo@...ngson.cn, zhenglifeng1@...wei.com,
joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
konrad.wilk@...cle.com
Subject: Re: [PATCH v10 00/11] arm64: support poll_idle()
Ankur Arora <ankur.a.arora@...cle.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
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