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

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Date:   Wed, 16 Oct 2019 16:32:21 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "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>,
        "wuyun.wu@...wei.com" <wuyun.wu@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH V2] arm64: psci: Reduce waiting time of
 cpu_psci_cpu_kill()

On Wed, Oct 09, 2019 at 12:45:16PM +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.
>
> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> ---
> V1->V2:
> - use usleep_range() instead of udelay() after waiting for a while
>
>  arch/arm64/kernel/psci.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> index c9f72b2..99b3122 100644
> --- a/arch/arm64/kernel/psci.c
> +++ b/arch/arm64/kernel/psci.c
> @@ -82,6 +82,7 @@ static void cpu_psci_cpu_die(unsigned int cpu)
>  static int cpu_psci_cpu_kill(unsigned int cpu)
>  {
>  	int err, i;
> +	unsigned long timeout;
>
>  	if (!psci_ops.affinity_info)
>  		return 0;
> @@ -91,16 +92,24 @@ static int cpu_psci_cpu_kill(unsigned int cpu)
>  	 * while it is dying. So, try again a few times.
>  	 */
>
> -	for (i = 0; i < 10; i++) {
> +	i = 0;
> +	timeout = jiffies + msecs_to_jiffies(100);
> +	do {
>  		err = psci_ops.affinity_info(cpu_logical_map(cpu), 0);
>  		if (err == PSCI_0_2_AFFINITY_LEVEL_OFF) {
>  			pr_info("CPU%d killed.\n", cpu);
>  			return 0;
>  		}
>
> -		msleep(10);
> -		pr_info("Retrying again to check for CPU kill\n");

You dropped this message, any particular reason ?

> -	}
> +		/* busy-wait max 1ms */
> +		if (i++ < 100) {
> +			cond_resched();
> +			udelay(10);
> +			continue;

Why can't it be simple like loop of 100 * msleep(1) instead of loop of
10 * msleep(10). The above initial busy wait for 1 ms looks too much
optimised for your setup where it takes 50-500us, what if it take just
over 1 ms ?

We need more generic solution.

--
Regards,
Sudeep