Subject: Re: [PATCH] arm64: psci: Use udelay() instead of msleep() to reduce waiting time

From: Yunfeng Ye <[email-masked]>
Date: Mon, 16 Sep 2019 23:26:20 +0800

On 2019/9/12 16:48, David Laight wrote:
> 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 <[email-masked]>
>> ---
>>  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
> 
Thanks for your advice. the delay depend on the num of cores, range
from 50us to 500us, I have test the time on the 140+ cores cpu：

  (10us every time)
  [ 1177.979642] psci: CPU1 killed. total wait 4 times
  [ 1178.011369] psci: CPU2 killed. total wait 6 times
  [ 1178.035247] psci: CPU3 killed. total wait 3 times
  [ 1178.071134] psci: CPU4 killed. total wait 8 times
  ......
  [ 1190.128202] psci: CPU139 killed. total wait 50 times
  [ 1190.156266] psci: CPU140 killed. total wait 48 times
  [ 1190.192082] psci: CPU141 killed. total wait 46 times
  [ 1190.224104] psci: CPU142 killed. total wait 46 times

Can I bust-wait 1ms，which is 100 tiems udelay(10), after that, use
usleep_range(1000, 10000) ?  I don't want other process occupy cpu
for a long time when I let out the cpu. thanks.

> -
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