Re: [PATCH v8 06/10] fs/resctrl: Add user interface to enable/disable io_alloc feature

["Gautham R. Shenoy" <gautham.shenoy@amd.com>]

Hello Babu,

On Tue, Aug 05, 2025 at 06:30:26PM -0500, 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.
> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>

[..snip..]


> +ssize_t resctrl_io_alloc_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;
> +	char const *grp_name;
> +	u32 io_alloc_closid;
> +	bool enable;
> +	int ret;
> +
> +	ret = kstrtobool(buf, &enable);
> +	if (ret)
> +		return ret;
> +
> +	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;
> +	}
> +
> +	io_alloc_closid = resctrl_io_alloc_closid(r);
> +	if (!resctrl_io_alloc_closid_supported(io_alloc_closid)) {
> +		rdt_last_cmd_printf("io_alloc CLOSID (ctrl_hw_id) %d is not available\n",
> +				    io_alloc_closid);
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
> +	/* If the feature is already up to date, no action is needed. */
> +	if (resctrl_arch_get_io_alloc_enabled(r) == enable)
> +		goto out_unlock;

Does it make sense to move this check before calling resctrl_io_alloc_closid(r) ?


> +
> +	if (enable) {
> +		if (!closid_alloc_fixed(io_alloc_closid)) {
> +			grp_name = rdtgroup_name_by_closid(io_alloc_closid);
> +			WARN_ON_ONCE(!grp_name);
> +			rdt_last_cmd_printf("CLOSID (ctrl_hw_id) %d for io_alloc is used by %s group\n",
> +					    io_alloc_closid, grp_name ? grp_name : "another");
> +			ret = -ENOSPC;
> +			goto out_unlock;
> +		}
> +
> +		ret = resctrl_io_alloc_init_cbm(s, io_alloc_closid);
> +		if (ret) {
> +			rdt_last_cmd_puts("Failed to initialize io_alloc allocations\n");
> +			closid_free(io_alloc_closid);
> +			goto out_unlock;
> +		}
> +	} else {
> +		closid_free(io_alloc_closid);
> +	}
> +
> +	ret = resctrl_arch_io_alloc_enable(r, enable);
> +
> +out_unlock:
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}

[..snip..]

-- 
Thanks and Regards
gautham.