Re: [PATCH v7 09/10] arm64: support cpuidle-haltpoll

[Will Deacon]

On Fri, Aug 30, 2024 at 03:28:43PM -0700, Ankur Arora wrote:
> Add architectural support for cpuidle-haltpoll driver by defining
> arch_haltpoll_*().
> 
> Also define ARCH_CPUIDLE_HALTPOLL to allow cpuidle-haltpoll to be
> selected, and given that we have an optimized polling mechanism
> in smp_cond_load*(), select ARCH_HAS_OPTIMIZED_POLL.
> 
> smp_cond_load*() are implemented via LDXR, WFE, with LDXR loading
> a memory region in exclusive state and the WFE waiting for any
> stores to it.
> 
> In the edge case -- no CPU stores to the waited region and there's no
> interrupt -- the event-stream will provide the terminating condition
> ensuring we don't wait forever, but because the event-stream runs at
> a fixed frequency (configured at 10kHz) we might spend more time in
> the polling stage than specified by cpuidle_poll_time().
> 
> This would only happen in the last iteration, since overshooting the
> poll_limit means the governor moves out of the polling stage.
> 
> Tested-by: Haris Okanovic <harisokn@xxxxxxxxxx>
> Tested-by: Misono Tomohiro <misono.tomohiro@xxxxxxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig                        | 10 ++++++++++
>  arch/arm64/include/asm/cpuidle_haltpoll.h | 10 ++++++++++
>  arch/arm64/kernel/Makefile                |  1 +
>  arch/arm64/kernel/cpuidle_haltpoll.c      | 22 ++++++++++++++++++++++
>  4 files changed, 43 insertions(+)
>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h
>  create mode 100644 arch/arm64/kernel/cpuidle_haltpoll.c
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a2f8ff354ca6..9bd93ce2f9d9 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -36,6 +36,7 @@ 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
> @@ -2385,6 +2386,15 @@ 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

nit: this 'default n' line is redundant.

> +	help
> +	  cpuidle-haltpoll allows for adaptive polling based on
> +	  current load before entering the idle state.
> +
> +	  Some virtualized workloads benefit from using it.

nit: This sentence is meaningless ^^.

> +
>  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..ed615a99803b
> --- /dev/null
> +++ b/arch/arm64/include/asm/cpuidle_haltpoll.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ARCH_HALTPOLL_H
> +#define _ARCH_HALTPOLL_H
> +
> +static inline void arch_haltpoll_enable(unsigned int cpu) { }
> +static inline void arch_haltpoll_disable(unsigned int cpu) { }
> +
> +bool arch_haltpoll_want(bool force);
> +#endif
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index 2b112f3b7510..bbfb57eda2f1 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -70,6 +70,7 @@ obj-$(CONFIG_ARM64_PTR_AUTH)		+= pointer_auth.o
>  obj-$(CONFIG_ARM64_MTE)			+= mte.o
>  obj-y					+= vdso-wrap.o
>  obj-$(CONFIG_COMPAT_VDSO)		+= vdso32-wrap.o
> +obj-$(CONFIG_ARCH_CPUIDLE_HALTPOLL)	+= cpuidle_haltpoll.o
>  
>  # Force dependency (vdso*-wrap.S includes vdso.so through incbin)
>  $(obj)/vdso-wrap.o: $(obj)/vdso/vdso.so
> diff --git a/arch/arm64/kernel/cpuidle_haltpoll.c b/arch/arm64/kernel/cpuidle_haltpoll.c
> new file mode 100644
> index 000000000000..63fc5ebca79b
> --- /dev/null
> +++ b/arch/arm64/kernel/cpuidle_haltpoll.c
> @@ -0,0 +1,22 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/kernel.h>
> +#include <clocksource/arm_arch_timer.h>
> +#include <asm/cpuidle_haltpoll.h>
> +
> +bool arch_haltpoll_want(bool force)
> +{
> +	/*
> +	 * Enabling haltpoll requires two things:
> +	 *
> +	 * - Event stream support to provide a terminating condition to the
> +	 *   WFE in the poll loop.
> +	 *
> +	 * - KVM support for arch_haltpoll_enable(), arch_haltpoll_disable().
> +	 *
> +	 * Given that the second is missing, allow haltpoll to only be force
> +	 * loaded.
> +	 */
> +	return (arch_timer_evtstrm_available() && false) || force;
> +}
> +EXPORT_SYMBOL_GPL(arch_haltpoll_want);

This seems a bit over-the-top to justify a new C file. Just have a static
inline in the header which returns 'force'. The '&& false' is misleading
and unnecessary with the comment.

Will