linux-kernel - Re: [PATCH v4 09/19] x86/resctrl: Initialize ABMC counters bitmap

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Date: Thu, 20 Jun 2024 15:20:47 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
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	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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	<eranian@...gle.com>, <james.morse@....com>
Subject: Re: [PATCH v4 09/19] x86/resctrl: Initialize ABMC counters bitmap

Hi Babu,

On 6/19/24 10:03 AM, Moger, Babu wrote:
> On 6/13/24 20:42, Reinette Chatre wrote:
>> On 5/24/24 5:23 AM, Babu Moger wrote:

...

>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index 3071bbb7a15e..400ae405e10e 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -186,6 +186,23 @@ bool closid_allocated(unsigned int closid)
>>>        return !test_bit(closid, &closid_free_map);
>>>    }
>>>    +/*
>>> + * ABMC Counter bitmap and length for tracking available counters.
>>> + * ABMC feature provides set of hardware counters for enabling events.
>>> + * Each event takes one hardware counter. Kernel needs to keep track
>>> + * of number of available counters.
>>> + */
>>> +static unsigned long num_cntrs_free_map;
>>
>> Why does variable have "num" in its name? That seems strange. How
>> about just "mon_cntrs_free_map
> 
> It came from patch 4/19.
> 
> struct resctrl_mon {
>          int                     num_rmid;
> +       int                     num_cntrs;
>          struct list_head        evt_list;
>   };
> 
> num_cntrs_free_map is a bitmap representing num_cntrs. Kept the matching
> name for better understanding. Renaming it will loose that connection.

I disagree. The "num" in "num_cntrs" indicates that this variable stores
the _number_ of a particular entity. In this case "cntrs" or ... counters.
This is just like how resctrl uses "num_closid" to indicate how many closid
are available and then have a separate "closid_free_map" to actually track
now closids are used ... it is _not_ "num_closid_free_map". Similarly,
"num_rmid" indicates how many RMID are available and the "rmid_free_lru"
tracks how RMID are used ... it is _not_ "num_rmid_free_lru".

> If I rename then I will have to rename both.

No, you do not.

> 
> How about mbm_cntrs and mbm_cntrs_free_map?

"mbm_cntrs" does sound good. It is more specific than "cntrs". I would suggest that
use "num_mbm_cntrs" to match with "num_rmid" and "num_closid" and then
you can introduce "mbm_cntrs_free_map".

>>> +static u32 num_cntrs_free_map_len;
>>
>> Same comment about "num" ... also, any special reason why u32 is needed?
> 
> Only reason is, it is supposed to be unsigned.  I can change it "unsigned
> int".
> 
>>
>>> +
>>> +static void num_cntrs_init(void)
>>
>> mon_cntrs_init() ?
> 
> mbm_cntrs_init?

Sounds good.

> 
>>
>>> +{
>>> +    struct rdt_resource *r =
>>> &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>> +
>>> +    bitmap_fill(&num_cntrs_free_map, r->mon.num_cntrs);
>>> +    num_cntrs_free_map_len = r->mon.num_cntrs;
>>> +}
>>> +
>>>    /**
>>>     * rdtgroup_mode_by_closid - Return mode of resource group with closid
>>>     * @closid: closid if the resource group
>>> @@ -2459,6 +2476,12 @@ static int resctrl_abmc_set_all(enum
>>> resctrl_res_level l, bool enable)
>>
>> resctrl_abmc_set_all() was initially created as a complement of
>> resctrl_abmc_set_one() ... but with more initialization added to
>> resctrl_abmc_set_all() this relationship becomes vague.
> 
> Yes. Understood. Let me know if want me to change anything here.

How about renaming resctrl_abmc_set_all() to _resctrl_abmc_enable()?

Reinette