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

[Rafael J. Wysocki]

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.

> 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,