Re: [PATCH v6 09/10] arm64: support cpuidle-haltpoll

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

On Fri, 2024-07-26 at 13:21 -0700, Ankur Arora wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> 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.
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>  arch/arm64/Kconfig                        | 10 ++++++++++
>  arch/arm64/include/asm/cpuidle_haltpoll.h |  9 +++++++++
>  arch/arm64/kernel/cpuidle.c               | 23 +++++++++++++++++++++++
>  3 files changed, 42 insertions(+)
>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5d91259ee7b5..cf1c6681eb0a 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -35,6 +35,7 @@ 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
> @@ -2376,6 +2377,15 @@ 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..65f289407a6c
> --- /dev/null
> +++ b/arch/arm64/include/asm/cpuidle_haltpoll.h
> @@ -0,0 +1,9 @@
> +/* 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/cpuidle.c b/arch/arm64/kernel/cpuidle.c
> index f372295207fb..334df82a0eac 100644
> --- a/arch/arm64/kernel/cpuidle.c
> +++ b/arch/arm64/kernel/cpuidle.c
> @@ -72,3 +72,26 @@ __cpuidle int acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi)
>                                              lpi->index, state);
>  }
>  #endif
> +
> +#if IS_ENABLED(CONFIG_HALTPOLL_CPUIDLE)
> +
> +#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_enable().

typo: "arch_haltpoll_enable" and "arch_haltpoll_enable"

> +        *
> +        * Given that the second is missing, allow haltpoll to only be force
> +        * loaded.
> +        */
> +       return (arch_timer_evtstrm_available() && false) || force;

This should always evaluate false without force. Perhaps you meant
something like this?

```
-       return (arch_timer_evtstrm_available() && false) || force;
+       return arch_timer_evtstrm_available() || force;
```

Regards,
Haris Okanovic

> +}
> +
> +EXPORT_SYMBOL_GPL(arch_haltpoll_want);
> +#endif
> --
> 2.43.5
>