Subject: Re: [PATCH v3 1/6] cpufreq: poowernv: Handle throttling due to Pmax capping at chip level

From: Preeti U Murthy <[email-masked]>
Date: Tue, 05 May 2015 14:08:32 +0530

On 05/05/2015 11:36 AM, Shilpasri G Bhat wrote:
> Hi Preeti,
> 
> On 05/05/2015 09:21 AM, Preeti U Murthy wrote:
>> Hi Shilpa,
>>
>> On 05/04/2015 02:24 PM, Shilpasri G Bhat wrote:
>>> The On-Chip-Controller(OCC) can throttle cpu frequency by reducing the
>>> max allowed frequency for that chip if the chip exceeds its power or
>>> temperature limits. As Pmax capping is a chip level condition report
>>> this throttling behavior at chip level and also do not set the global
>>> 'throttled' on Pmax capping instead set the per-chip throttled
>>> variable. Report unthrottling if Pmax is restored after throttling.
>>>
>>> This patch adds a structure to store chip id and throttled state of
>>> the chip.
>>>
>>> Signed-off-by: Shilpasri G Bhat <[email-masked]>
>>> ---
>>>  drivers/cpufreq/powernv-cpufreq.c | 59 ++++++++++++++++++++++++++++++++++++---
>>>  1 file changed, 55 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
>>> index ebef0d8..d0c18c9 100644
>>> --- a/drivers/cpufreq/powernv-cpufreq.c
>>> +++ b/drivers/cpufreq/powernv-cpufreq.c
>>> @@ -27,6 +27,7 @@
>>>  #include <linux/smp.h>
>>>  #include <linux/of.h>
>>>  #include <linux/reboot.h>
>>> +#include <linux/slab.h>
>>>
>>>  #include <asm/cputhreads.h>
>>>  #include <asm/firmware.h>
>>> @@ -42,6 +43,13 @@
>>>  static struct cpufreq_frequency_table powernv_freqs[POWERNV_MAX_PSTATES+1];
>>>  static bool rebooting, throttled;
>>>
>>> +static struct chip {
>>> +	unsigned int id;
>>> +	bool throttled;
>>> +} *chips;
>>> +
>>> +static int nr_chips;
>>> +
>>>  /*
>>>   * Note: The set of pstates consists of contiguous integers, the
>>>   * smallest of which is indicated by powernv_pstate_info.min, the
>>> @@ -301,22 +309,33 @@ static inline unsigned int get_nominal_index(void)
>>>  static void powernv_cpufreq_throttle_check(unsigned int cpu)
>>>  {
>>>  	unsigned long pmsr;
>>> -	int pmsr_pmax, pmsr_lp;
>>> +	int pmsr_pmax, pmsr_lp, i;
>>>
>>>  	pmsr = get_pmspr(SPRN_PMSR);
>>>
>>> +	for (i = 0; i < nr_chips; i++)
>>> +		if (chips[i].id == cpu_to_chip_id(cpu))
>>> +			break;
>>> +
>>>  	/* Check for Pmax Capping */
>>>  	pmsr_pmax = (s8)PMSR_MAX(pmsr);
>>>  	if (pmsr_pmax != powernv_pstate_info.max) {
>>> -		throttled = true;
>>> -		pr_info("CPU %d Pmax is reduced to %d\n", cpu, pmsr_pmax);
>>> -		pr_info("Max allowed Pstate is capped\n");
>>> +		if (chips[i].throttled)
>>> +			goto next;
>>> +		chips[i].throttled = true;
>>> +		pr_info("CPU %d on Chip %u has Pmax reduced to %d\n", cpu,
>>> +			chips[i].id, pmsr_pmax);
>>> +	} else if (chips[i].throttled) {
>>> +		chips[i].throttled = false;
>>
>> Is this check on pmax sufficient to indicate that the chip is unthrottled ?
> 
> Unthrottling due to Pmax uncapping here is specific to a chip. So it is
> sufficient to decide throttling/unthrottling when OCC is active for that chip.

Ok then we can perhaps exit after detecting unthrottling here.
> 
>>
>>> +		pr_info("CPU %d on Chip %u has Pmax restored to %d\n", cpu,
>>> +			chips[i].id, pmsr_pmax);
>>>  	}
>>>
>>>  	/*
>>>  	 * Check for Psafe by reading LocalPstate
>>>  	 * or check if Psafe_mode_active is set in PMSR.
>>>  	 */
>>> +next:
>>>  	pmsr_lp = (s8)PMSR_LP(pmsr);
>>>  	if ((pmsr_lp < powernv_pstate_info.min) ||
>>>  				(pmsr & PMSR_PSAFE_ENABLE)) {
>>> @@ -414,6 +433,33 @@ static struct cpufreq_driver powernv_cpufreq_driver = {
>>>  	.attr		= powernv_cpu_freq_attr,
>>
>> What about the situation where although occ is active, this particular
>> chip has been throttled and we end up repeatedly reporting "pstate set
>> to safe" and "frequency control disabled from OS" ? Should we not have a
>> check on (chips[i].throttled) before reporting an anomaly for these two
>> scenarios as well just like you have for pmsr_pmax ?
> 
> We will not have "Psafe" and "frequency control disabled" repeatedly printed
> because of global variable 'throttled', which is set to true on passing any of
> these two conditions.
> 
> It is quite unlikely behavior to have only one chip in "Psafe" or "frequency
> control disabled" state. These two conditions are most likely to happen during
> an OCC reset cycle which will occur across all chips.

Let us then add a comment to indicate that Psafe and frequency control
disabled conditions will fail *only if OCC is inactive* and not
otherwise and that this is a system wide phenomenon.

Regards
Preeti U Murthy

> 
> Thanks and Regards,
> Shilpa
> 
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