linux-kernel - Re: [PATCH] intel_pstate: Do not skip samples partially

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Date:	Thu, 10 Mar 2016 14:53:46 -0800
From:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Philippe Longepe <philippe.longepe@...ux.intel.com>
Subject: Re: [PATCH] intel_pstate: Do not skip samples partially

On Thu, 2016-03-10 at 23:45 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> If the current value of MPERF or the current value of TSC is the
> same as the previous one, respectively, intel_pstate_sample() bails
> out early and skips the sample.
> 
> However, intel_pstate_adjust_busy_pstate() is still called in that
> case which is not correct, so modify intel_pstate_sample() to
> return a bool value indicating whether or not the sample has been
> taken and use it to decide whether or not to call
> intel_pstate_adjust_busy_pstate().
> 
> While at it, remove redundant parentheses from the MPERF/TSC
> check in intel_pstate_sample().
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> ---
> 
> On top of my linux-next branch.
> 
> ---
>  drivers/cpufreq/intel_pstate.c |   12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -890,7 +890,7 @@ static inline void intel_pstate_calc_bus
>  	sample->core_pct_busy = (int32_t)core_pct;
>  }
>  
> -static inline void intel_pstate_sample(struct cpudata *cpu, u64
> time)
> +static inline bool intel_pstate_sample(struct cpudata *cpu, u64
> time)
>  {
>  	u64 aperf, mperf;
>  	unsigned long flags;
> @@ -900,9 +900,9 @@ static inline void intel_pstate_sample(s
>  	rdmsrl(MSR_IA32_APERF, aperf);
>  	rdmsrl(MSR_IA32_MPERF, mperf);
>  	tsc = rdtsc();
> -	if ((cpu->prev_mperf == mperf) || (cpu->prev_tsc == tsc)) {
> +	if (cpu->prev_mperf == mperf || cpu->prev_tsc == tsc) {
>  		local_irq_restore(flags);
> -		return;
> +		return false;
>  	}
>  	local_irq_restore(flags);
>  
> @@ -920,6 +920,7 @@ static inline void intel_pstate_sample(s
>  	cpu->prev_aperf = aperf;
>  	cpu->prev_mperf = mperf;
>  	cpu->prev_tsc = tsc;
> +	return true;
>  }
>  
>  static inline int32_t get_target_pstate_use_cpu_load(struct cpudata
> *cpu)
> @@ -1026,8 +1027,9 @@ static void intel_pstate_update_util(str
>  	u64 delta_ns = time - cpu->sample.time;
>  
>  	if ((s64)delta_ns >= pid_params.sample_rate_ns) {
> -		intel_pstate_sample(cpu, time);
> -		if (!hwp_active)
> +		bool sample_taken = intel_pstate_sample(cpu, time);
> +
> +		if (sample_taken && !hwp_active)
>  			intel_pstate_adjust_busy_pstate(cpu);
>  	}
>  }
> 
> --
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