linux-kernel - Re: [PATCH v8 06/10] fs/resctrl: Add user interface to enable/disable io

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Message-ID: <d5438a53-c803-4704-84db-1da019f50a3d@amd.com>
Date: Fri, 22 Aug 2025 17:53:10 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, Dave.Martin@....com, james.morse@....com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, akpm@...ux-foundation.org,
 paulmck@...nel.org, rostedt@...dmis.org, Neeraj.Upadhyay@....com,
 david@...hat.com, arnd@...db.de, fvdl@...gle.com, seanjc@...gle.com,
 thomas.lendacky@....com, pawan.kumar.gupta@...ux.intel.com,
 yosry.ahmed@...ux.dev, sohil.mehta@...el.com, xin@...or.com,
 kai.huang@...el.com, xiaoyao.li@...el.com, peterz@...radead.org,
 me@...aill.net, mario.limonciello@....com, xin3.li@...el.com,
 ebiggers@...gle.com, ak@...ux.intel.com, chang.seok.bae@...el.com,
 andrew.cooper3@...rix.com, perry.yuan@....com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, manali.shukla@....com, gautham.shenoy@....com
Subject: Re: [PATCH v8 06/10] fs/resctrl: Add user interface to enable/disable
 io_alloc feature

Hi Reinette,

On 8/7/2025 8:49 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 8/5/25 4:30 PM, Babu Moger wrote:
>> "io_alloc" feature in resctrl enables direct insertion of data from I/O
>> devices into the cache.
>>
>> On AMD systems, when io_alloc is enabled, the highest CLOSID is reserved
>> exclusively for I/O allocation traffic and is no longer available for
>> general CPU cache allocation. Users are encouraged to enable it only when
>> running workloads that can benefit from this functionality.
>>
>> Since CLOSIDs are managed by resctrl fs, it is least invasive to make the
>> "io_alloc is supported by maximum supported CLOSID" part of the initial
>> resctrl fs support for io_alloc. Take care not to expose this use of CLOSID
>> for io_alloc to user space so that this is not required from other
>> architectures that may support io_alloc differently in the future.
>>
>> Introduce user interface to enable/disable io_alloc feature.
> Please include high level overview of what this patch does to enable
> and disable io_alloc. Doing so will help connect why the changelog contains
> information about CLOSID management.


Sure.

>
>> diff --git a/fs/resctrl/ctrlmondata.c b/fs/resctrl/ctrlmondata.c
>> index d495a5d5c9d5..bf982eab7b18 100644
>> --- a/fs/resctrl/ctrlmondata.c
>> +++ b/fs/resctrl/ctrlmondata.c
>> @@ -685,3 +685,140 @@ int resctrl_io_alloc_show(struct kernfs_open_file *of, struct seq_file *seq, voi
>>   
>>   	return 0;
>>   }
>> +
>> +/*
>> + * resctrl_io_alloc_closid_supported() - io_alloc feature utilizes the
>> + * highest CLOSID value to direct I/O traffic. Ensure that io_alloc_closid
>> + * is in the supported range.
>> + */
>> +static bool resctrl_io_alloc_closid_supported(u32 io_alloc_closid)
>> +{
>> +	return io_alloc_closid < closids_supported();
>> +}
>> +
>> +static struct resctrl_schema *resctrl_get_schema(enum resctrl_conf_type type)
>> +{
>> +	struct resctrl_schema *schema;
>> +
>> +	list_for_each_entry(schema, &resctrl_schema_all, list) {
>> +		if (schema->conf_type == type)
>> +			return schema;
> This does not look right. More than one resource can have the same configuration type, no?
> Think about L2 and L3 having CDP enabled ...
> Looks like this is missing a resource type as parameter and a check for the resource ...
> but is this function even necessary (more below)?

May not be required.  Comments below.

>
>> +	}
>> +
>> +	return NULL;
>> +}
>> +
>> +/*
>> + * Initialize io_alloc CLOSID cache resource CBM with all usable (shared
>> + * and unused) cache portions.
>> + */
>> +static int resctrl_io_alloc_init_cbm(struct resctrl_schema *s, u32 closid)
>> +{
>> +	struct rdt_resource *r = s->res;
> Needs reverse fir.


Sure.

>
>> +	enum resctrl_conf_type peer_type;
>> +	struct resctrl_schema *peer_s;
>> +	int ret;
>> +
>> +	rdt_staged_configs_clear();
>> +
>> +	ret = rdtgroup_init_cat(s, closid);
>> +	if (ret < 0)
>> +		goto out;
>> +
>> +	/* Initialize schema for both CDP_DATA and CDP_CODE when CDP is enabled */
>> +	if (resctrl_arch_get_cdp_enabled(r->rid)) {
>> +		peer_type = resctrl_peer_type(s->conf_type);
>> +		peer_s = resctrl_get_schema(peer_type);
>> +		if (peer_s) {
>> +			ret = rdtgroup_init_cat(peer_s, closid);
> This is unexpected. In v7 I suggested that when parsing the CBM of one of the CDP
> resources it is not necessary to do so again for the peer. The CBM can be
> parsed *once* and the configuration just copied over. See:
> https://lore.kernel.org/lkml/82045638-2b26-4682-9374-1c3e400a580a@intel.com/

Let met try to understand.

So, rdtgroup_init_cat() sets up the staged _config for the specific CDP 
type for all the domains.

We need to apply those staged_configs to its peer type on all the domains.

Something like this?

/* Initialize staged_config of the peer type when CDP is enabled */
         if (resctrl_arch_get_cdp_enabled(r->rid)) {
                 list_for_each_entry(d, &s->res->ctrl_domains, hdr.list) {
                         cfg = &d->staged_config[s->conf_type];
                         cfg_peer = &d->staged_config[peer_type];
                         cfg_peer->new_ctrl = cfg->new_ctrl;
                         cfg_peer->have_new_ctrl = cfg->have_new_ctrl;
                 }
         }


>
> Generally when feedback is provided it is good to check all places in series where
> it is relevant. oh ... but looking ahead you ignored the feedback in the patch
> it was given also :(


My bad.

I will address that.

Thanks

Babu