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

["Moger, Babu" <babu.moger@amd.com>]

Hi Gautham,

On 8/21/2025 12:02 AM, Gautham R. Shenoy wrote:
> 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) ?


Sure. We can do that.

Thanks

Babu


>
>
>> +
>> +	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..]
>