linux-kernel - Re: [PATCH V2] arm64: psci: Reduce waiting time of cpu_psci_cpu

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Date:   Wed, 16 Oct 2019 11:25:45 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     Will Deacon <will@...nel.org>,
        David Laight <David.Laight@...LAB.COM>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "kstewart@...uxfoundation.org" <kstewart@...uxfoundation.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "wuyun.wu@...wei.com" <wuyun.wu@...wei.com>, hushiyuan@...wei.com,
        linfeilong@...wei.com
Subject: Re: [PATCH V2] arm64: psci: Reduce waiting time of
 cpu_psci_cpu_kill()

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.

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.

> 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 ?

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

Thanks Will.

--
Regards,
Sudeep