linux-kernel - Re: [PATCH v10 10/11] arm64: idle: export arch_cpu

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Message-ID: <87h62u76xg.fsf@oracle.com>
Date: Fri, 11 Apr 2025 13:57:15 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-pm@...r.kernel.org,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, x86@...nel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, peterz@...radead.org, arnd@...db.de,
        lenb@...nel.org, mark.rutland@....com, harisokn@...zon.com,
        mtosatti@...hat.com, sudeep.holla@....com, cl@...two.org,
        maz@...nel.org, misono.tomohiro@...itsu.com, maobibo@...ngson.cn,
        zhenglifeng1@...wei.com, joao.m.martins@...cle.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v10 10/11] arm64: idle: export arch_cpu_idle()


Shuai Xue <xueshuai@...ux.alibaba.com> writes:

> 在 2025/2/19 05:33, Ankur Arora 写道:
>> Needed for cpuidle-haltpoll.
>> Acked-by: Will Deacon <will@...nel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>   arch/arm64/kernel/idle.c | 1 +
>>   1 file changed, 1 insertion(+)
>> diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
>> index 05cfb347ec26..b85ba0df9b02 100644
>> --- a/arch/arm64/kernel/idle.c
>> +++ b/arch/arm64/kernel/idle.c
>> @@ -43,3 +43,4 @@ void __cpuidle arch_cpu_idle(void)
>>   	 */
>>   	cpu_do_idle();
>
> Hi, Ankur,
>
> With haltpoll_driver registered, arch_cpu_idle() on x86 can select
> mwait_idle() in idle threads.
>
> It use MONITOR sets up an effective address range that is monitored
> for write-to-memory activities; MWAIT places the processor in
> an optimized state (this may vary between different implementations)
> until a write to the monitored address range occurs.

MWAIT is more capable than WFE -- it allows selection of deeper idle
state. IIRC C2/C3.

> Should arch_cpu_idle() on arm64 also use the LDXR/WFE
> to avoid wakeup IPI like x86 monitor/mwait?

Avoiding the wakeup IPI needs TIF_NR_POLLING and polling in idle support
that this series adds.

As Haris notes, the negative with only using WFE is that it only allows
a single idle state, one that is fairly shallow because the event-stream
causes a wakeup every 100us.

--
ankur