Re: [PATCH V2] arm64: psci: Reduce waiting time of cpu_psci_cpu_kill()

[Yunfeng Ye <yeyunfeng@huawei.com>]

On 2019/10/16 18:25, Sudeep Holla wrote:
> On Wed, Oct 16, 2019 at 11:22:23AM +0800, Yunfeng Ye wrote:
>>
>>
>> On 2019/10/16 0:23, Will Deacon wrote:
>>> Hi,
>>>
>>> On Sat, Sep 21, 2019 at 07:21:17PM +0800, Yunfeng Ye wrote:
>>>> If psci_ops.affinity_info() fails, it will sleep 10ms, which will not
>>>> take so long in the right case. Use usleep_range() instead of msleep(),
>>>> reduce the waiting time, and give a chance to busy wait before sleep.
>>>
>>> Can you elaborate on "the right case" please? It's not clear to me
>>> exactly what problem you're solving here.
>>>
>> The situation is that when the power is off, we have a battery to save some
>> information, but the battery power is limited, so we reduce the power consumption
>> by turning off the cores, and need fastly to complete the core shutdown. However, the
>> time of cpu_psci_cpu_kill() will take 10ms. We have tested the time that it does not
>> need 10ms, and most case is about 50us-500us. if we reduce the time of cpu_psci_cpu_kill(),
>> we can reduce 10% - 30% of the total time.
>>
> 
> Have you checked why PSCI AFFINITY_INFO not returning LEVEL_OFF quickly
> then ? We wait for upto 5s in cpu_wait_death(worst case) before cpu_kill
> is called from __cpu_die.
> 
When cpu_wait_death() is done, it means that the cpu core's hardware prepare to
die. I think not returning LEVEL_OFF quickly is that hardware need time to handle.
I don't know how much time it need is reasonable, but I test that it need about
50us - 500us.

In addition I have not meat the worst case that cpu_wait_death() need upto 5s, and
we only take normal case into account.

thanks.

> Moreover I don't understand the argument here. The cpu being killed
> will be OFF, as soon as it can and firmware controls that and this
> change is not related to CPU_OFF. And this CPU calling cpu_kill can
> sleep and 10ms is good to enter idle states if it's idle saving power,
> so I fail to map the power saving you mention above.
> 
We have hundreds of CPU cores that need to be shut down. For example,
a CPU has 200 cores, and the thread to shut down the core is in CPU 0.
and the thread need to shut down from core 1 to core 200. However, the
implementation of the kernel can only shut down cpu cores one by one, so we
need to wait for cpu_kill() to finish before shutting down the next
CPU core. If it wait for 10ms each time in cpu_kill, it will takes up
about 2 seconds in cpu_kill() total.

It is not to save power through msleep to idle state, but to quickly
turn off other CPU core's hardware to reduce power consumption.

thanks.

>> So change msleep (10) to usleep_range() to reduce the waiting time. In addition,
>> we don't want to be scheduled during the sleeping time, some threads may take a
>> long time and don't give up the CPU, which affects the time of core shutdown,
>> Therefore, we add a chance to busy-wait max 1ms.
>>
> 
> On the other hand, usleep_range reduces the timer interval and hence
> increases the chance of the callee CPU not to enter deeper idle states.
> 
> What am I missing here ? What's the use case or power off situation
> you are talking about above ?
> 
As mentioned above, we are not to save power through msleep to idle state,  but to quickly
turn off other CPU core's hardware to reduce power consumption.

>>
>>> I've also added Sudeep to the thread, since I'd like his ack on the change.
>>>
> 
> Thanks Will.
> 
> --
> Regards,
> Sudeep
> 
> .
>