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

[Jinjie Ruan]

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,
-- 
2.34.1