linux-kernel - Re: [PATCH v4] arm64: psci: Reduce the waiting time for cpu_psci_cpu

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Date:   Fri, 18 Oct 2019 16:20:52 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        kstewart@...uxfoundation.org, gregkh@...uxfoundation.org,
        lorenzo.pieralisi@....com, tglx@...utronix.de,
        David.Laight@...LAB.COM, mark.rutland@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        hushiyuan@...wei.com, wuyun.wu@...wei.com, linfeilong@...wei.com
Subject: Re: [PATCH v4] arm64: psci: Reduce the waiting time for
 cpu_psci_cpu_kill()

On Fri, Oct 18, 2019 at 08:46:37PM +0800, Yunfeng Ye wrote:
> In case like suspend-to-disk and uspend-to-ram, a large number of CPU

s/case/cases/
s/uspend-to-ram/suspend-to-ram/

> cores need to be shut down. At present, the CPU hotplug operation is
> serialised, and the CPU cores can only be shut down one by one. In this
> process, if PSCI affinity_info() does not return LEVEL_OFF quickly,
> cpu_psci_cpu_kill() needs to wait for 10ms. If hundreds of CPU cores
> need to be shut down, it will take a long time.
> 
> Normally, there is no need to wait 10ms in cpu_psci_cpu_kill(). So
> change the wait interval from 10 ms to max 1 ms and use usleep_range()
> instead of msleep() for more accurate timer.
> 
> In addition, reducing the time interval will increase the messages
> output, so remove the "Retry ..." message, instead, put the number of
> waiting times to the sucessful message.
> 
> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> ---
> v3 -> v4:
>  - using time_before(jiffies, timeout) to check
>  - update the comment as review suggest
> 
> v2 -> v3:
>  - update the comment
>  - remove the busy-wait logic, modify the loop logic and output message
> 
> v1 -> v2:
>  - use usleep_range() instead of udelay() after waiting for a while
>  arch/arm64/kernel/psci.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> index c9f72b2665f1..77965c3ba477 100644
> --- a/arch/arm64/kernel/psci.c
> +++ b/arch/arm64/kernel/psci.c
> @@ -81,7 +81,8 @@ static void cpu_psci_cpu_die(unsigned int cpu)
> 
>  static int cpu_psci_cpu_kill(unsigned int cpu)
>  {
> -	int err, i;
> +	int err, i = 0;
> +	unsigned long timeout;
> 
>  	if (!psci_ops.affinity_info)
>  		return 0;
> @@ -91,16 +92,17 @@ 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++) {
> +	timeout = jiffies + msecs_to_jiffies(100);
> +	do {
> +		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);
> +			pr_info("CPU%d killed (polled %d times)\n", cpu, i);

We can even drop loop counter completely, track time and log that
instead of loop counter that doesn't give any indication without looking
into the code.

	start = jiffies, end = start + msecs_to_jiffies(100);
	do {
			....
			pr_info("CPU%d killed (polled %u ms)\n", cpu,
				jiffies_to_msecs(jiffies - start));
			....
	} while (time_before(jiffies, end));

Just my preference. Looks good otherwise.

--
Regards,
Sudeep