Re: [PATCH v2 2/4] cpufreq: intel_pstate: Avoid missing HWP max updates in passive mode

[Zhang Rui <rui.zhang@intel.com>]

On Fri, 2020-10-23 at 17:35 +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the cpufreq policy max limit is changed when intel_pstate operates
> in the passive mode with HWP enabled and the "powersave" governor is
> used on top of it, the HWP max limit is not updated as appropriate.
> 
> Namely, in the "powersave" governor case, the target P-state
> is always equal to the policy min limit, so if the latter does
> not change, intel_cpufreq_adjust_hwp() is not invoked to update
> the HWP Request MSR due to the "target_pstate != old_pstate" check
> in intel_cpufreq_update_pstate(), so the HWP max limit is not
> updated as a result.
> 
> Also, if the CPUFREQ_NEED_UPDATE_LIMITS flag is not set for the
> driver and the target frequency does not change along with the
> policy max limit, the "target_freq == policy->cur" check in
> __cpufreq_driver_target() prevents the driver's ->target() callback
> from being invoked at all, so the HWP max limit is not updated.
> 
> To prevent that occurring, set the CPUFREQ_NEED_UPDATE_LIMITS flag
> in the intel_cpufreq driver structure if HWP is enabled and modify
> intel_cpufreq_update_pstate() to do the "target_pstate != old_pstate"
> check only in the non-HWP case and let intel_cpufreq_adjust_hwp()
> always run in the HWP case (it will update HWP Request only if the
> cached value of the register is different from the new one including
> the limits, so if neither the target P-state value nor the max limit
> changes, the register write will still be avoided).
> 
> Fixes: f6ebbcf08f37 ("cpufreq: intel_pstate: Implement passive mode
> with HWP enabled")
> Reported-by: Zhang Rui <rui.zhang@intel.com>
> Cc: 5.9+ <stable@vger.kernel.org> # 5.9+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

I have confirmed that the problem is gone with this patch series
applied.
The HWP register is updated after changing the scaling_max_freq sysfs
attribute, with powersave governor.

Tested-by: Zhang Rui <rui.zhang@intel.com>

thanks,
rui
> ---
> 
> The v2 is just the intel_pstate changes (without the core changes)
> and setting
> the new flag.
> 
> ---
>  drivers/cpufreq/intel_pstate.c |   13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -2550,14 +2550,12 @@ static int intel_cpufreq_update_pstate(s
>  	int old_pstate = cpu->pstate.current_pstate;
>  
>  	target_pstate = intel_pstate_prepare_request(cpu,
> target_pstate);
> -	if (target_pstate != old_pstate) {
> +	if (hwp_active) {
> +		intel_cpufreq_adjust_hwp(cpu, target_pstate,
> fast_switch);
> +		cpu->pstate.current_pstate = target_pstate;
> +	} else if (target_pstate != old_pstate) {
> +		intel_cpufreq_adjust_perf_ctl(cpu, target_pstate,
> fast_switch);
>  		cpu->pstate.current_pstate = target_pstate;
> -		if (hwp_active)
> -			intel_cpufreq_adjust_hwp(cpu, target_pstate,
> -						 fast_switch);
> -		else
> -			intel_cpufreq_adjust_perf_ctl(cpu,
> target_pstate,
> -						      fast_switch);
>  	}
>  
>  	intel_cpufreq_trace(cpu, fast_switch ?
> INTEL_PSTATE_TRACE_FAST_SWITCH :
> @@ -3014,6 +3012,7 @@ static int __init intel_pstate_init(void
>  			hwp_mode_bdw = id->driver_data;
>  			intel_pstate.attr = hwp_cpufreq_attrs;
>  			intel_cpufreq.attr = hwp_cpufreq_attrs;
> +			intel_cpufreq.flags |=
> CPUFREQ_NEED_UPDATE_LIMITS;
>  			if (!default_driver)
>  				default_driver = &intel_pstate;
>  
> 
> 
>