linux-kernel - Re: [PATCH RESEND v4 1/4] cpufreq: powernv: Remove cpu_to_chip

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Date:	Tue, 12 Jan 2016 16:27:37 +0530
From:	Shreyas B Prabhu <shreyas@...ux.vnet.ibm.com>
To:	Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>
CC:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	rjw@...ysocki.net, viresh.kumar@...aro.org,
	linux-pm@...r.kernel.org, pc@...ibm.com, anton@...ba.org,
	ego@...ux.vnet.ibm.com
Subject: Re: [PATCH RESEND v4 1/4] cpufreq: powernv: Remove cpu_to_chip_id()
 from hot-path



On 01/12/2016 03:54 PM, Shilpasri G Bhat wrote:
> cpu_to_chip_id() does a DT walk through to find out the chip id by taking a
> contended device tree lock. This adds an unnecessary overhead in a hot-path.
> So instead of cpu_to_chip_id() use PIR of the cpu to find the chip id.
> 
> Reported-by: Anton Blanchard <anton@...ba.org>
> Signed-off-by: Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>
> ---
>  drivers/cpufreq/powernv-cpufreq.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
> index cb50138..597a084 100644
> --- a/drivers/cpufreq/powernv-cpufreq.c
> +++ b/drivers/cpufreq/powernv-cpufreq.c
> @@ -39,6 +39,7 @@
>  #define PMSR_PSAFE_ENABLE	(1UL << 30)
>  #define PMSR_SPR_EM_DISABLE	(1UL << 31)
>  #define PMSR_MAX(x)		((x >> 32) & 0xFF)
> +#define pir_to_chip_id(pir)	(((pir) >> 7) & 0x3f)

Since this is platform specific and true only for power8, this is not
the right place to put it. Either you can move this to arch/powerpc  or
you can maintain a cpu to chip map within the driver.
> 
>  static struct cpufreq_frequency_table powernv_freqs[POWERNV_MAX_PSTATES+1];
>  static bool rebooting, throttled, occ_reset;
> @@ -312,13 +313,14 @@ static inline unsigned int get_nominal_index(void)
>  static void powernv_cpufreq_throttle_check(void *data)
>  {
>  	unsigned int cpu = smp_processor_id();
> +	unsigned int chip_id = pir_to_chip_id(hard_smp_processor_id());
>  	unsigned long pmsr;
>  	int pmsr_pmax, i;
> 
>  	pmsr = get_pmspr(SPRN_PMSR);
> 
>  	for (i = 0; i < nr_chips; i++)
> -		if (chips[i].id == cpu_to_chip_id(cpu))
> +		if (chips[i].id == chip_id)
>  			break;
> 
>  	/* Check for Pmax Capping */
> @@ -558,7 +560,8 @@ static int init_chip_info(void)
>  	unsigned int prev_chip_id = UINT_MAX;
> 
>  	for_each_possible_cpu(cpu) {
> -		unsigned int id = cpu_to_chip_id(cpu);
> +		unsigned int id =
> +			pir_to_chip_id(get_hard_smp_processor_id(cpu));
> 
>  		if (prev_chip_id != id) {
>  			prev_chip_id = id;
> 

Thanks,
Shreyas