Re: [PATCH v8 09/10] fs/resctrl: Introduce interface to modify io_alloc Capacity Bit Masks

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Babu,

On 8/5/25 4:30 PM, Babu Moger wrote:
> "io_alloc" feature enables direct insertion of data from I/O devices into
> the cache. By directly caching data from I/O devices rather than first
> storing the I/O data in DRAM, it reduces the demands on DRAM bandwidth and
> reduces latency to the processor consuming the I/O data.
> 
> When CDP is enabled, io_alloc routes traffic using the highest CLOSID
> associated with the L3CODE resource. To ensure consistent cache allocation
> behavior, the L3CODE and L3DATA resources must remain synchronized.
> rdtgroup_init_cat() function takes both L3CODE and L3DATA into account when
> initializing CBMs for new groups.  The io_alloc feature must maintain the
> same behavior, ensuring that the Cache Bit Masks (CBMs) for both L3CODE and
> L3DATA are updated together.

Please rework this copy&pasted text also and make specific to what this
patch actually does.

> Enable users to modify io_alloc CBMs (Capacity Bit Masks) via the
> io_alloc_cbm resctrl file when io_alloc is enabled.

Here the changelog can provide overview of what is done by this patch
when a user provides a new CBM. This can include that a CBM provided
to CDP enabled resource will copy the CBM to the CDP peer.

> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> ---



> ---
>  Documentation/filesystems/resctrl.rst |  8 +++
>  fs/resctrl/ctrlmondata.c              | 97 +++++++++++++++++++++++++++
>  fs/resctrl/internal.h                 |  3 +
>  fs/resctrl/rdtgroup.c                 |  3 +-
>  4 files changed, 110 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index 3002f7fdb2fe..d955e8525af0 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -187,6 +187,14 @@ related to allocation:
>  			# cat /sys/fs/resctrl/info/L3/io_alloc_cbm
>  			0=ffff;1=ffff
>  
> +		CBMs can be configured by writing to the interface.
> +
> +		Example::
> +
> +			# echo 1=FF > /sys/fs/resctrl/info/L3/io_alloc_cbm

It may be useful to demonstrate syntax when more than one CBM is modified.

> +			# cat /sys/fs/resctrl/info/L3/io_alloc_cbm
> +			0=ffff;1=00ff
> +
>  		When CDP is enabled "io_alloc_cbm" associated with the DATA and CODE
>  		resources may reflect the same values. For example, values read from and
>  		written to /sys/fs/resctrl/info/L3DATA/io_alloc_cbm may be reflected by
> diff --git a/fs/resctrl/ctrlmondata.c b/fs/resctrl/ctrlmondata.c
> index 641094aac322..1f69117f96f8 100644
> --- a/fs/resctrl/ctrlmondata.c
> +++ b/fs/resctrl/ctrlmondata.c
> @@ -858,3 +858,100 @@ int resctrl_io_alloc_cbm_show(struct kernfs_open_file *of, struct seq_file *seq,
>  	cpus_read_unlock();
>  	return ret;
>  }
> +
> +static int resctrl_io_alloc_parse_line(char *line,  struct rdt_resource *r,
> +				       struct resctrl_schema *s, u32 closid)
> +{
> +	enum resctrl_conf_type peer_type;
> +	struct resctrl_schema *peer_s;
> +	struct rdt_parse_data data;
> +	struct rdt_ctrl_domain *d;
> +	char *dom = NULL, *id;
> +	unsigned long dom_id;
> +
> +next:
> +	if (!line || line[0] == '\0')
> +		return 0;
> +
> +	dom = strsep(&line, ";");
> +	id = strsep(&dom, "=");
> +	if (!dom || kstrtoul(id, 10, &dom_id)) {
> +		rdt_last_cmd_puts("Missing '=' or non-numeric domain\n");
> +		return -EINVAL;
> +	}
> +
> +	dom = strim(dom);
> +	list_for_each_entry(d, &r->ctrl_domains, hdr.list) {
> +		if (d->hdr.id == dom_id) {
> +			data.buf = dom;
> +			data.mode = RDT_MODE_SHAREABLE;
> +			data.closid = closid;
> +			if (parse_cbm(&data, s, d))
> +				return -EINVAL;
> +			/*
> +			 * When CDP is enabled, update the schema for both CDP_DATA
> +			 * and CDP_CODE.
> +			 */
> +			if (resctrl_arch_get_cdp_enabled(r->rid)) {
> +				peer_type = resctrl_peer_type(s->conf_type);
> +				peer_s = resctrl_get_schema(peer_type);
> +				if (parse_cbm(&data, peer_s, d))
> +					return -EINVAL;

CBM is still parsed twice. As I mentioned in v7 the parsing only need to be done
once and then the resulting CBM can be copied to CDP peer.
https://lore.kernel.org/lkml/82045638-2b26-4682-9374-1c3e400a580a@intel.com/

> +			}
> +			goto next;
> +		}
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +ssize_t resctrl_io_alloc_cbm_write(struct kernfs_open_file *of, char *buf,
> +				   size_t nbytes, loff_t off)
> +{
> +	struct resctrl_schema *s = rdt_kn_parent_priv(of->kn);
> +	struct rdt_resource *r = s->res;
> +	u32 io_alloc_closid;
> +	int ret = 0;
> +
> +	/* Valid input requires a trailing newline */
> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
> +		return -EINVAL;
> +
> +	buf[nbytes - 1] = '\0';
> +
> +	cpus_read_lock();
> +	mutex_lock(&rdtgroup_mutex);
> +
> +	rdt_last_cmd_clear();
> +
> +	if (!r->cache.io_alloc_capable) {
> +		rdt_last_cmd_printf("io_alloc is not supported on %s\n", s->name);
> +		ret = -ENODEV;
> +		goto out_unlock;
> +	}
> +
> +	rdt_last_cmd_clear();

Unnecessary rdt_last_cmd_clear().

> +	rdt_staged_configs_clear();

Placement of this can be improved by putting it closer to the code that
populates the staged configs. That is, just before resctrl_io_alloc_parse_line().
The flow is not symmetrical in that the out_unlock exit code always
clears the buffer whether it was used or not. I think it will be easier to
understand if the out_unlock *only* unlocks the locks and there is a new
goto label, for example, "out_clear_configs" that calls rdt_staged_configs_clear()
and is used after resctrl_io_alloc_parse_line().

> +
> +	if (!resctrl_arch_get_io_alloc_enabled(r)) {
> +		rdt_last_cmd_printf("io_alloc is not enabled on %s\n", s->name);
> +		ret = -ENODEV;
> +		goto out_unlock;
> +	}
> +
> +	io_alloc_closid = resctrl_io_alloc_closid(r);
> +
> +	ret = resctrl_io_alloc_parse_line(buf, r, s, io_alloc_closid);
> +
> +	if (ret)
> +		goto out_unlock;
> +
> +	ret = resctrl_arch_update_domains(r, io_alloc_closid);
> +
> +out_unlock:
> +	rdt_staged_configs_clear();
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}

Reinette