Re: [PATCH v10 10/11] arm64: idle: export arch_cpu_idle()

[Shuai Xue <xueshuai@linux.alibaba.com>]

在 2025/4/14 11:46, Ankur Arora 写道:
> 
> Shuai Xue <xueshuai@linux.alibaba.com> writes:
> 
>> 在 2025/4/12 04:57, Ankur Arora 写道:
>>> Shuai Xue <xueshuai@linux.alibaba.com> writes:
>>>
>>>> 在 2025/2/19 05:33, Ankur Arora 写道:
>>>>> Needed for cpuidle-haltpoll.
>>>>> Acked-by: Will Deacon <will@kernel.org>
>>>>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.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
>>
>> Hi, Ankur and Haris
>>
>> Got it, thanks for explaination :)
>>
>> Comparing sched-pipe performance on Rund with Yitian 710, *IPC improved 35%*:
> 
> Thanks for testing Shuai. I wasn't expecting the IPC to improve by quite
> that much :). The reduced instructions make sense since we don't have to
> handle the IRQ anymore but we would spend some of the saved cycles
> waiting in WFE instead.
> 
> I'm not familiar with the Yitian 710. Can you check if you are running
> with WFE? That's the __smp_cond_load_relaxed_timewait() path vs the
> __smp_cond_load_relaxed_spinwait() path in [0]. Same question for the
> Kunpeng 920.

Yes, it running with __smp_cond_load_relaxed_timewait().

I use perf-probe to check if WFE is available in Guest:

perf probe 'arch_timer_evtstrm_available%return r=$retval'
perf record -e probe:arch_timer_evtstrm_available__return -aR sleep 1
perf script
swapper       0 [000]  1360.063049: probe:arch_timer_evtstrm_available__return: (ffff800080a5c640 <- ffff800080d42764) r=0x1

arch_timer_evtstrm_available returns true, so
__smp_cond_load_relaxed_timewait() is used.

> 
> Also, I'm working on a new version of the series in [1]. Would you be
> okay trying that out?

Sure. Please cc me when you send out a new version.

> 
> Thanks
> Ankur
> 
> [0] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
> [1] https://lore.kernel.org/lkml/20250203214911.898276-4-ankur.a.arora@oracle.com/
> 

Thanks.
Shuai