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

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Date:   Thu, 17 Oct 2019 21:26:15 +0800
From:   Yunfeng Ye <yeyunfeng@...wei.com>
To:     Sudeep Holla <sudeep.holla@....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>, <hushiyuan@...wei.com>,
        <linfeilong@...wei.com>
Subject: Re: [PATCH V2] arm64: psci: Reduce waiting time of
 cpu_psci_cpu_kill()



On 2019/10/16 23:32, Sudeep Holla wrote:
> 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 ?
> 
When reduce the time interval to 1ms, the print message maybe increase 10 times.
on the other hand, cpu_psci_cpu_kill() will print message on success or failure, which
this retry log is not very necessary. of cource, I think use pr_info_once() instead of
pr_info() is better.

thanks.

>> -	}
>> +		/* 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 ?
> 
msleep() is implemented by jiffies. when HZ=100 or HZ=250, msleep(1) is not
accurate. so I think usleep_range() is better. 1 ms looks simple and good, but how
about 100us is better? I refer a function sunxi_mc_smp_cpu_kill(), it use
usleep_range(50, 100).

thanks.

> We need more generic solution.
> 
> --
> Regards,
> Sudeep
> 
> .
>