linux-kernel - Re: [PATCH v3 1/8] x86/resctrl: Refactor in preparation for node-scoped resources

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Date:   Tue, 18 Jul 2023 13:36:26 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        "Peter Newman" <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, <x86@...nel.org>
CC:     Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <patches@...ts.linux.dev>
Subject: Re: [PATCH v3 1/8] x86/resctrl: Refactor in preparation for
 node-scoped resources

Hi Tony,

On 7/13/2023 9:32 AM, Tony Luck wrote:
> Sub-NUMA cluster systems provide monitoring resources at the NUMA
> node scope instead of the L3 cache scope.
> 
> Rename the cache_level field in struct rdt_resource to the more
> generic "scope" and add symbolic names and a helper function.

Can the changelog elaborate how the helper function is intended
to be used? When changelog just states "add a helper function" it
is unnecessary since that is clear from the code.

> 
> No functional change.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> Reviewed-by: Peter Newman <peternewman@...gle.com>
> ---

...

> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 030d3b409768..6571514752f3 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -65,7 +65,7 @@ struct rdt_hw_resource rdt_resources_all[] = {
>  		.r_resctrl = {
>  			.rid			= RDT_RESOURCE_L3,
>  			.name			= "L3",
> -			.cache_level		= 3,
> +			.scope			= SCOPE_L3_CACHE,
>  			.domains		= domain_init(RDT_RESOURCE_L3),
>  			.parse_ctrlval		= parse_cbm,
>  			.format_str		= "%d=%0*x",
> @@ -79,7 +79,7 @@ struct rdt_hw_resource rdt_resources_all[] = {
>  		.r_resctrl = {
>  			.rid			= RDT_RESOURCE_L2,
>  			.name			= "L2",
> -			.cache_level		= 2,
> +			.scope			= SCOPE_L2_CACHE,
>  			.domains		= domain_init(RDT_RESOURCE_L2),
>  			.parse_ctrlval		= parse_cbm,
>  			.format_str		= "%d=%0*x",
> @@ -93,7 +93,7 @@ struct rdt_hw_resource rdt_resources_all[] = {
>  		.r_resctrl = {
>  			.rid			= RDT_RESOURCE_MBA,
>  			.name			= "MB",
> -			.cache_level		= 3,
> +			.scope			= SCOPE_L3_CACHE,
>  			.domains		= domain_init(RDT_RESOURCE_MBA),
>  			.parse_ctrlval		= parse_bw,
>  			.format_str		= "%d=%*u",
> @@ -105,7 +105,7 @@ struct rdt_hw_resource rdt_resources_all[] = {
>  		.r_resctrl = {
>  			.rid			= RDT_RESOURCE_SMBA,
>  			.name			= "SMBA",
> -			.cache_level		= 3,
> +			.scope			= 3,

Should this be SCOPE_L3_CACHE?

>  			.domains		= domain_init(RDT_RESOURCE_SMBA),
>  			.parse_ctrlval		= parse_bw,
>  			.format_str		= "%d=%*u",

...

> diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> index 458cb7419502..42f124ffb968 100644
> --- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> +++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> @@ -297,7 +297,7 @@ static int pseudo_lock_region_init(struct pseudo_lock_region *plr)
>  	plr->size = rdtgroup_cbm_to_size(plr->s->res, plr->d, plr->cbm);
>  
>  	for (i = 0; i < ci->num_leaves; i++) {
> -		if (ci->info_list[i].level == plr->s->res->cache_level) {
> +		if (ci->info_list[i].level == plr->s->res->scope) {
>  			plr->line_size = ci->info_list[i].coherency_line_size;
>  			return 0;
>  		}
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 725344048f85..418658f0a9ad 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1348,7 +1348,7 @@ unsigned int rdtgroup_cbm_to_size(struct rdt_resource *r,
>  	num_b = bitmap_weight(&cbm, r->cache.cbm_len);
>  	ci = get_cpu_cacheinfo(cpumask_any(&d->cpu_mask));
>  	for (i = 0; i < ci->num_leaves; i++) {
> -		if (ci->info_list[i].level == r->cache_level) {
> +		if (ci->info_list[i].level == r->scope) {
>  			size = ci->info_list[i].size / r->cache.cbm_len * num_b;
>  			break;
>  		}

The last two hunks are red flags to me. Clearly the "cache_level"->"scope"
change is done in preparation for "scope" to be assigned more values than
2 or 3. Yet the code continue to use these values as cache levels, comparing
it to cacheinfo->level for which I only expect cache levels 2 or 3 to be valid.
The above two hunks thus now have potential for errors when rdt_resource->scope
has a value that is not 2 or 3. 

Even if these functions may not be called if rdt_resource->scope is not 2 or 3,
this change makes the code harder to understand and maintain because now it
requires users to know in which flows particular functions can be called and/or
when code paths with invalid values are "ok".

Reinette