linux-kernel - Re: [PATCH v7 05/12] x86/resctrl: Detect and configure Slow Memory Bandwidth allocation

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Date:   Thu, 27 Oct 2022 11:37:46 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     <babu.moger@....com>, <corbet@....net>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <bp@...en8.de>
CC:     <fenghua.yu@...el.com>, <dave.hansen@...ux.intel.com>,
        <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
        <akpm@...ux-foundation.org>, <quic_neeraju@...cinc.com>,
        <rdunlap@...radead.org>, <damien.lemoal@...nsource.wdc.com>,
        <songmuchun@...edance.com>, <peterz@...radead.org>,
        <jpoimboe@...nel.org>, <pbonzini@...hat.com>,
        <chang.seok.bae@...el.com>, <pawan.kumar.gupta@...ux.intel.com>,
        <jmattson@...gle.com>, <daniel.sneddon@...ux.intel.com>,
        <sandipan.das@....com>, <tony.luck@...el.com>,
        <james.morse@....com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <bagasdotme@...il.com>,
        <eranian@...gle.com>
Subject: Re: [PATCH v7 05/12] x86/resctrl: Detect and configure Slow Memory
 Bandwidth allocation

Hi Babu,

On 10/27/2022 8:30 AM, Moger, Babu wrote:
> On 10/26/22 15:23, Reinette Chatre wrote:
>> On 10/26/2022 12:07 PM, Moger, Babu wrote:
>>> On 10/25/22 18:43, Reinette Chatre wrote:
>>>> On 10/17/2022 3:26 PM, Babu Moger wrote:
>>>>
>>>> ...
>>>>
>>>>> @@ -2845,7 +2846,8 @@ static int rdtgroup_init_alloc(struct rdtgroup *rdtgrp)
>>>>>  
>>>>>  	list_for_each_entry(s, &resctrl_schema_all, list) {
>>>>>  		r = s->res;
>>>>> -		if (r->rid == RDT_RESOURCE_MBA) {
>>>>> +		if (r->rid == RDT_RESOURCE_MBA ||
>>>>> +		    r->rid == RDT_RESOURCE_SMBA) {
>>>>>  			rdtgroup_init_mba(r, rdtgrp->closid);
>>>>>  			if (is_mba_sc(r))
>>>>>  				continue;
>>>> The above hunk and the ones that follow are unexpected.
>>> I am thinking the above check is required, It is updating the
>>> staged_config with default values. Right now, the default value for SMBA
>>> is same as MBA default value. So, I used this code to initialize.
>>>
>>> Did I miss something?
>> As I described in the following comments my concern is related to all the
>> software controller code still executing for SMBA. Yes, in the above hunk
>> SMBA would need (some of) rdtgroup_init_mba() ... but note that it contains
>> software controller checks and in the above hunk its call is also followed
>> by another software controller check.
>>
>> The software controller is just applicable to MBA and these checks have been
>> isolated to the MBA resource. Using it for SMBA that does not support
>> software controller at all is making the code harder to follow and sets this
>> code up for future mistakes. I think it would make the code easier to understand
>> if this is made very clear that software controller is not applicable to SMBA at
>> all instead of repurposing these flows.
> 
> Yes. Understood.  How about this? I feel this is much more cleaner.
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index e5a48f05e787..d91a6a513681 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -2845,16 +2845,18 @@ static int rdtgroup_init_alloc(struct rdtgroup
> *rdtgrp)
>  
>         list_for_each_entry(s, &resctrl_schema_all, list) {
>                 r = s->res;
> -               if (r->rid == RDT_RESOURCE_MBA) {
> +               if (r->rid == RDT_RESOURCE_MBA ||
> +                   r->rid == RDT_RESOURCE_SMBA) {
>                         rdtgroup_init_mba(r, rdtgrp->closid);
> -                       if (is_mba_sc(r))
> -                               continue;
>                 } else {
>                         ret = rdtgroup_init_cat(s, rdtgrp->closid);
>                         if (ret < 0)
>                                 return ret;
>                 }
>  
> +               if (is_mba_sc(r))
> +                       continue;
> +
>                 ret = resctrl_arch_update_domains(r, rdtgrp->closid);
>                 if (ret < 0) {
>                         rdt_last_cmd_puts("Failed to initialize
> allocations\n");
> 

I do not see how that move changes what is run in the SMBA case and it ignores the
is_mba_sc() call within rdtgroup_init_mba(). How about making is_mba_sc() more
robust in support of your original snippet?

Something like:

bool is_mba_sc(struct rdt_resource *r)
{
	if (!r)
		return rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl.membw.mba_sc;

	if (r->rid != RDT_RESOURCE_MBA)
		return false;

	return r->membw.mba_sc;
}

Reinette