linux-kernel - RE: [PATCH] arm64: psci: Use udelay() instead of msleep() to reduce waiting time

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Date:   Thu, 12 Sep 2019 08:48:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Yunfeng Ye' <yeyunfeng@...wei.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>
CC:     "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>
Subject: RE: [PATCH] arm64: psci: Use udelay() instead of msleep() to reduce
 waiting time

From: Yunfeng Ye
> Sent: 11 September 2019 09:50
> We want to reduce the time of cpu_down() for saving power, found that
> cpu_psci_cpu_kill() cost 10ms after psci_ops.affinity_info() fail.
> 
> Normally the time cpu dead is very short, it is no need to wait 10ms.
> so use udelay 10us to instead msleep 10ms in every waiting loop, and add
> cond_resched() to give a chance to run a higher-priority process.
> 
> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> ---
>  arch/arm64/kernel/psci.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> index 85ee7d0..9e9d8a6 100644
> --- a/arch/arm64/kernel/psci.c
> +++ b/arch/arm64/kernel/psci.c
> @@ -86,15 +86,15 @@ 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++) {
> +	for (i = 0; i < 10000; i++) {
>  		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");
> +		cond_resched();
> +		udelay(10);

You really don't want to be doing 10000 udelay(10) before giving up.

If udelay(10) is long enough for the normal case, then do that once.
After that use usleep_range().

	David

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