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

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

maobibo <maobibo@loongson.cn> writes:

> On 2024/7/27 上午4:21, 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.
>> 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) { }
> It is better that guest supports halt poll on more architectures, LoongArch
> wants this if result is good.
>
> Do we need disable halt polling on host hypervisor if guest also uses halt
> polling idle method?

Yes. The intent is to work on that separately from this series. As the comment
below states, until that is available we only allow force loading.

>> +
>> +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().
>> +	 *
>> +	 * 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);
>> +#endif
>>


--
ankur