Re: [PATCH v8 11/11] arm64: support cpuidle-haltpoll

[Ankur Arora <ankur.a.arora@oracle.com>]

Okanovic, Haris <harisokn@amazon.com> writes:

> On Wed, 2024-09-25 at 16:24 -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 the cpuidle-haltpoll driver by defining
>> arch_haltpoll_*(). Also define ARCH_CPUIDLE_HALTPOLL to allow
>> cpuidle-haltpoll to be selected.
>>
>> Haltpoll uses poll_idle() to do the actual polling. This in turn
>> uses smp_cond_load*() to wait until there's a specific store to
>> a cacheline.
>> In the edge case -- no stores to the cacheline and no interrupt --
>> the event-stream provides the terminating condition ensuring we
>> don't wait forever. But because the event-stream runs at a fixed
>> frequency (configured at 10kHz) haltpoll 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 will move out of the polling stage.
>>
>> Tested-by: Haris Okanovic <harisokn@amazon.com>
>> Tested-by: Misono Tomohiro <misono.tomohiro@fujitsu.com>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>  arch/arm64/Kconfig                        |  6 ++++++
>>  arch/arm64/include/asm/cpuidle_haltpoll.h | 24 +++++++++++++++++++++++
>>  2 files changed, 30 insertions(+)
>>  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index ef9c22c3cff2..5fc99eba22b2 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -2415,6 +2415,12 @@ config ARCH_HIBERNATION_HEADER
>>  config ARCH_SUSPEND_POSSIBLE
>>         def_bool y
>>
>> +config ARCH_CPUIDLE_HALTPOLL
>> +       bool "Enable selection of the cpuidle-haltpoll driver"
>> +       help
>> +         cpuidle-haltpoll allows for adaptive polling based on
>> +         current load before entering the idle state.
>> +
>>  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..91f0be707629
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/cpuidle_haltpoll.h
>> @@ -0,0 +1,24 @@
>> +/* 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) { }
>> +
>> +static inline 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, only allow force loading for
>> +        * haltpoll.
>> +        */
>> +       return force;
>> +}
>> +#endif
>> --
>> 2.43.5
>>
>
> I applied your patches to master e32cde8d2bd7 and verified same
> performance gains on AWS Graviton.

Great.

> Reviewed-by: Haris Okanovic <harisokn@amazon.com>
> Tested-by: Haris Okanovic <harisokn@amazon.com>

Thanks!

--
ankur