Re: [PATCH 8/9] arm64: support cpuidle-haltpoll

["Okanovic, Haris" <harisokn@amazon.com>]

On Tue, 2024-04-30 at 11:37 -0700, Ankur Arora wrote:
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> 
> 
> 
> Add architectural support for the cpuidle-haltpoll driver by defining
> arch_haltpoll_*(). Also select ARCH_HAS_OPTIMIZED_POLL since we have
> an optimized polling mechanism via smp_cond_load*().
> 
> Add the configuration option, ARCH_CPUIDLE_HALTPOLL to allow
> cpuidle-haltpoll to be selected.
> 
> Note that we limit cpuidle-haltpoll support to when the event-stream is
> available. This is necessary because polling via smp_cond_load_relaxed()
> uses WFE to wait for a store which might not happen for an prolonged
> period of time. So, ensure the event-stream is around to provide a
> terminating condition.
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>  arch/arm64/Kconfig                        | 10 ++++++++++
>  arch/arm64/include/asm/cpuidle_haltpoll.h | 21 +++++++++++++++++++++
>  2 files changed, 31 insertions(+)
>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7b11c98b3e84..6f2df162b10e 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -34,6 +34,7 @@ config ARM64
>         select ARCH_HAS_MEMBARRIER_SYNC_CORE
>         select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
>         select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> +       select ARCH_HAS_OPTIMIZED_POLL
>         select ARCH_HAS_PTE_DEVMAP
>         select ARCH_HAS_PTE_SPECIAL
>         select ARCH_HAS_HW_PTE_YOUNG
> @@ -2331,6 +2332,15 @@ config ARCH_HIBERNATION_HEADER
>  config ARCH_SUSPEND_POSSIBLE
>         def_bool y
> 
> +config ARCH_CPUIDLE_HALTPOLL
> +       bool "Enable selection of the cpuidle-haltpoll driver"
> +       default n
> +       help
> +         cpuidle-haltpoll allows for adaptive polling based on
> +         current load before entering the idle state.
> +
> +         Some virtualized workloads benefit from using it.
> +
>  endmenu # "Power management options"
> 
>  menu "CPU Power Management"
> diff --git a/arch/arm64/include/asm/cpuidle_haltpoll.h b/arch/arm64/include/asm/cpuidle_haltpoll.h
> new file mode 100644
> index 000000000000..a79bdec7f516
> --- /dev/null
> +++ b/arch/arm64/include/asm/cpuidle_haltpoll.h
> @@ -0,0 +1,21 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_HALTPOLL_H
> +#define _ASM_HALTPOLL_H
> +
> +static inline void arch_haltpoll_enable(unsigned int cpu)
> +{
> +}
> +
> +static inline void arch_haltpoll_disable(unsigned int cpu)
> +{
> +}
> +
> +static inline bool arch_haltpoll_supported(void)
> +{
> +       /*
> +        * Ensure the event stream is available to provide a terminating
> +        * condition to the WFE in the poll loop.
> +        */
> +       return arch_timer_evtstrm_available();

Note this fails build when CONFIG_HALTPOLL_CPUIDLE=m (module):

ERROR: modpost: "arch_cpu_idle" [drivers/cpuidle/cpuidle-haltpoll.ko]
undefined!                                                         
ERROR: modpost: "arch_timer_evtstrm_available"
[drivers/cpuidle/cpuidle-haltpoll.ko] undefined!                      
make[2]: *** [scripts/Makefile.modpost:145: Module.symvers] Error 1
make[1]: *** [/home/ubuntu/linux/Makefile:1886: modpost] Error 2
make: *** [Makefile:240: __sub-make] Error 2

You could add EXPORT_SYMBOL_*()'s on the above helpers or restrict
HALTPOLL_CPUIDLE module to built-in (remove "tristate" Kconfig).

Otherwise, everything worked for me when built-in (=y) atop 6.10.0
(4a4be1a). I see similar performance gains in `perf bench` on AWS
Graviton3 c7g.16xlarge.

Regards,
Haris Okanovic

> +}
> +#endif
> --
> 2.39.3
> 

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