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

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Message-ID: <ef734fc1-8a51-4c60-f73f-2d71fcc54b30@amd.com>
Date:   Wed, 26 Oct 2022 14:07:43 -0500
From:   "Moger, Babu" <babu.moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.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 Reinette,

On 10/25/22 18:43, Reinette Chatre wrote:
> Hi Babu,
>
> Nitpick in Subject ... "allocation" -> "Allocation"?
Sure.
>
> 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?

>
> Note that the software controller, when resctrl is mounted with "mba_MBps", is 
> only supported by RDT_RESOURCE_MBA. At this time this really is hard coded all
> over the place, for example:
>
> static int set_mba_sc(bool mba_sc)
> {
> 	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
> 	...
>
> }
>
> Since SMBA hardcodes "delay_linear = false" I do not expect it to support the software
> controller ... but these hunks appear to treat SMBA as though it does. It is the "MBA software
> controller", not "SMBA software controller". Why does it check above if the MBA software
> controller is enabled on SMBA?

There is no plan to support SMBA software controller. Yes, I think below
checks are not required.


> 			
>
>> @@ -3287,7 +3289,8 @@ void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d)
>>  {
>>  	lockdep_assert_held(&rdtgroup_mutex);
>>  
>> -	if (supports_mba_mbps() && r->rid == RDT_RESOURCE_MBA)
>> +	if (supports_mba_mbps() &&
>> +	    (r->rid == RDT_RESOURCE_MBA || r->rid == RDT_RESOURCE_SMBA))
>>  		mba_sc_domain_destroy(r, d);
This check is not required.
>>  
>>  	if (!r->mon_capable)
>> @@ -3354,8 +3357,9 @@ int resctrl_online_domain(struct rdt_resource *r, struct rdt_domain *d)
>>  
>>  	lockdep_assert_held(&rdtgroup_mutex);
>>  
>> -	if (supports_mba_mbps() && r->rid == RDT_RESOURCE_MBA)
>> -		/* RDT_RESOURCE_MBA is never mon_capable */
>> +	if (supports_mba_mbps() &&
>> +	    (r->rid == RDT_RESOURCE_MBA || r->rid == RDT_RESOURCE_MBA))
>> +		/* RDT_RESOURCE_MBA (or SMBA) is never mon_capable */
> What does this change do? Did you mean to add a r->rid == RDT_RESOURCE_SMBA check?

Good catch. I meant  r->rid == RDT_RESOURCE_SMBA.

But this check is not required at all.

>
>>  		return mba_sc_domain_allocate(r, d);
>>  
>>  	if (!r->mon_capable)
>>
>>
> Why are the MBA software controller resources allocated/destroyed for a SMBA resource? If
> you want to support the software controller for SMBA then there are a lot of other changes

No..There is no plan to support software controller for SMBA.

Thanks

Babu