Re: [PATCH] cpufreq: intel_pstate: Fix CPU lowest Frequency bug when offline/online for passive

[Jinjie Ruan]

On 2023/11/21 1:16, Rafael J. Wysocki wrote:
> On Mon, Nov 13, 2023 at 7:18 AM Jinjie Ruan <ruanjinjie@xxxxxxxxxx> wrote:
>>
>> Ping.
> 
> I see the problem, but I'm not sure if the approach taken in the patch
> is the best one, as it has side effects.

Could you please make it more clear, what are the side effects? I'm not
sure about the possible negative effects of the patch.

> 
>> On 2023/11/7 10:58, Jinjie Ruan wrote:
>>> There is a bug in passive mode for intel pstate when
>>> CONFIG_X86_INTEL_PSTATE = y and configure intel_pstate = passive in command
>>> line. On Ice Lake server, although the performance tuner is used, the CPU
>>> have the lowest frequency in scaling_cur_freq after the CPU goes offline and
>>> then goes online, running the same infinite loop load.
>>>
>>> How to reproduce:
>>>
>>> cat while_true.c
>>>       #include <stdio.h>
>>>       void main(void)
>>>       {
>>>               while(1);
>>>       }
>>>
>>> [root@localhost freq_test]# cat test.sh
>>>       #!/bin/bash
>>>
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_governor
>>>       taskset -c ${1} ./while_true &
>>>       sleep 1s
>>>
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>>       echo 0 > /sys/devices/system/cpu/cpu${1}/online
>>>
>>>       sleep 1s
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>>       sleep 1s
>>>
>>>       echo 1 > /sys/devices/system/cpu/cpu${1}/online
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>>       taskset -c ${1} ./while_true &
>>>
>>>       sleep 1s
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>>       sleep 1s
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>>       sleep 1s
>>>       cat /sys/devices/system/cpu/cpu${1}/cpufreq/scaling_cur_freq
>>>
>>> The CPU frequency is adjusted to the minimum after offline and online:
>>>
>>> [root@localhost freq_test]# sh test.sh 20
>>>       2300000
>>>       performance
>>>       2299977
>>>       cat: /sys/devices/system/cpu/cpu20/cpufreq/scaling_cur_freq: Device or
>>>       resource busy
>>>       800000
>>>       800000
>>>       800000
>>>       799992
>>> [root@localhost freq_test]# sh test.sh 21
>>>       2300000
>>>       performance
>>>       2300000
>>>       cat: /sys/devices/system/cpu/cpu21/cpufreq/scaling_cur_freq: Device or
>>>       resource busy
>>>       800000
>>>       800000
>>>       800000
>>>       800000
>>>
>>> As in __cpufreq_driver_target(), the cpufreq core will not call intel
>>> cpufreq's target() callback if the target freq is equal with policy->cur
>>> and do not set CPUFREQ_NEED_UPDATE_LIMITS flag, but the hardware also not
>>> proactively keep CPU with the policy->cur frequency. So also set
>>> CPUFREQ_NEED_UPDATE_LIMITS for passive mode. After applying this patch,
>>> the CPU frequency is consistent as what performance tuner expected after
>>> CPU offline and online as below:
>>>
>>> [root@localhost freq_test]# sh test.sh 20
>>>       2300000
>>>       performance
>>>       2300000
>>>       cat: /sys/devices/system/cpu/cpu20/cpufreq/scaling_cur_freq: Device or resource busy
>>>       2300000
>>>       2300000
>>>       2299977
>>>       2299977
>>> [root@localhost freq_test]# sh test.sh 21
>>>       2300000
>>>       performance
>>>       2300000
>>>       cat: /sys/devices/system/cpu/cpu21/cpufreq/scaling_cur_freq: Device or resource busy
>>>       2300000
>>>       2300000
>>>       2300000
>>>       2300000
>>> [root@localhost freq_test]# cat /sys/devices/system/cpu/intel_pstate/status
>>>       passive
>>>
>>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
>>> ---
>>>  drivers/cpufreq/intel_pstate.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
>>> index a534a1f7f1ee..73403f1292b0 100644
>>> --- a/drivers/cpufreq/intel_pstate.c
>>> +++ b/drivers/cpufreq/intel_pstate.c
>>> @@ -3091,7 +3091,7 @@ static int intel_cpufreq_suspend(struct cpufreq_policy *policy)
>>>  }
>>>
>>>  static struct cpufreq_driver intel_cpufreq = {
>>> -     .flags          = CPUFREQ_CONST_LOOPS,
>>> +     .flags          = CPUFREQ_CONST_LOOPS | CPUFREQ_NEED_UPDATE_LIMITS,
>>>       .verify         = intel_cpufreq_verify_policy,
>>>       .target         = intel_cpufreq_target,
>>>       .fast_switch    = intel_cpufreq_fast_switch,
>