Re: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm_all interface for CBM reset

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

Hi Aaron,

Thanks for trying the patches.

On 11/6/2025 7:24 PM, Aaron Tomlin wrote:
> This patch introduces the new resctrl interface file "io_alloc_min_cbm_all"
> to provide users with a clean mechanism to reset all I/O allocation CBMs
> (Cache-Block Masks) to their minimum configuration.
> 
> Writing '0' to this file triggers the logic to set each corresponding CBM
> to the minimum number of consecutive bits (effectively clearing them to 0
> or their smallest supported mask). This simplifies the process of clearing
> or resetting the I/O allocation state without requiring manual CBM string
> calculations and eliminating the need for multiple writes to "io_alloc_cbm".

Your patch replaces the need for

[1] # echo "0=0;1=0" > /sys/fs/resctrl/info/L3/io_alloc_cbm

Instead it does

[2] # echo 0 > /sys/fs/resctrl/info/L3/io_alloc_min_cbm_all

Both approaches involve a single system call, with no multiple writes in 
either case.

I don’t see a strong justification for keeping both options. Option [1] 
is consistent with the existing interfaces and is also efficient, so I 
don’t see the need for an additional mechanism.


> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>   Documentation/filesystems/resctrl.rst     |  13 +++
>   arch/x86/kernel/cpu/resctrl/core.c        |   2 +-
>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  23 +++--
>   fs/resctrl/ctrlmondata.c                  | 117 ++++++++++++++++++----
>   fs/resctrl/internal.h                     |   3 +
>   fs/resctrl/rdtgroup.c                     |  10 +-
>   include/linux/resctrl.h                   |  30 +++++-
>   7 files changed, 165 insertions(+), 33 deletions(-)
> 
> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index ccc425b65b27..9899bc716459 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -213,6 +213,19 @@ related to allocation:
>   		written to /sys/fs/resctrl/info/L3DATA/io_alloc_cbm may be reflected by
>   		/sys/fs/resctrl/info/L3CODE/io_alloc_cbm and vice versa.
>   
> +"io_alloc_min_cbm_all":
> +		Set each CBM to their minimum number of consecutive bits.
> +
> +		Example::
> +
> +			# cat /sys/fs/resctrl/info/L3/io_alloc_cbm
> +			0=ffff;1=00ff
> +
> +			# echo 0 > /sys/fs/resctrl/info/L3/io_alloc_min_cbm_all
> +
> +			# cat /sys/fs/resctrl/info/L3/io_alloc_cbm
> +			0=0;1=0
> +
>   Memory bandwidth(MB) subdirectory contains the following files
>   with respect to allocation:
>   
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 3792ab4819dc..44aea6b534e0 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -276,7 +276,7 @@ static void rdt_get_cdp_config(int level)
>   
>   static void rdt_set_io_alloc_capable(struct rdt_resource *r)
>   {
> -	r->cache.io_alloc_capable = true;
> +	r->cache.io_alloc.io_alloc_capable = true;
>   }
>   
>   static void rdt_get_cdp_l3_config(void)
> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index b20e705606b8..0f051d848422 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -57,14 +57,19 @@ int resctrl_arch_update_domains(struct rdt_resource *r, u32 closid)
>   		hw_dom = resctrl_to_arch_ctrl_dom(d);
>   		msr_param.res = NULL;
>   		for (t = 0; t < CDP_NUM_TYPES; t++) {
> -			cfg = &hw_dom->d_resctrl.staged_config[t];
> -			if (!cfg->have_new_ctrl)
> -				continue;
> -
> -			idx = resctrl_get_config_index(closid, t);
> -			if (cfg->new_ctrl == hw_dom->ctrl_val[idx])
> -				continue;
> -			hw_dom->ctrl_val[idx] = cfg->new_ctrl;
> +			if (resctrl_should_io_alloc_min_cbm(r)) {
> +				idx = resctrl_get_config_index(closid, t);
> +				hw_dom->ctrl_val[idx] = apply_io_alloc_min_cbm(r);
> +			} else {
> +				cfg = &hw_dom->d_resctrl.staged_config[t];
> +				if (!cfg->have_new_ctrl)
> +					continue;
> +
> +				idx = resctrl_get_config_index(closid, t);
> +				if (cfg->new_ctrl == hw_dom->ctrl_val[idx])
> +					continue;
> +				hw_dom->ctrl_val[idx] = cfg->new_ctrl;
> +			}
>   
>   			if (!msr_param.res) {
>   				msr_param.low = idx;
> @@ -123,7 +128,7 @@ int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable)
>   {
>   	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>   
> -	if (hw_res->r_resctrl.cache.io_alloc_capable &&
> +	if (hw_res->r_resctrl.cache.io_alloc.io_alloc_capable &&
>   	    hw_res->sdciae_enabled != enable) {
>   		_resctrl_sdciae_enable(r, enable);
>   		hw_res->sdciae_enabled = enable;
> diff --git a/fs/resctrl/ctrlmondata.c b/fs/resctrl/ctrlmondata.c
> index b2d178d3556e..6cbf4cfaf974 100644
> --- a/fs/resctrl/ctrlmondata.c
> +++ b/fs/resctrl/ctrlmondata.c
> @@ -688,7 +688,7 @@ int resctrl_io_alloc_show(struct kernfs_open_file *of, struct seq_file *seq, voi
>   
>   	mutex_lock(&rdtgroup_mutex);
>   
> -	if (r->cache.io_alloc_capable) {
> +	if (r->cache.io_alloc.io_alloc_capable) {
>   		if (resctrl_arch_get_io_alloc_enabled(r))
>   			seq_puts(seq, "enabled\n");
>   		else
> @@ -758,6 +758,50 @@ u32 resctrl_io_alloc_closid(struct rdt_resource *r)
>   		return resctrl_arch_get_num_closid(r) - 1;
>   }
>   
> +/*
> + * check_io_alloc_support() - Establish if io_alloc is supported
> + *
> + * @s: resctrl resource schema.
> + *
> + * This function must be called under the cpu hotplug lock
> + * and rdtgroup mutex
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +static int check_io_alloc_support(struct resctrl_schema *s)
> +{
> +	struct rdt_resource *r = s->res;
> +
> +	if (!r->cache.io_alloc.io_alloc_capable) {
> +		rdt_last_cmd_printf("io_alloc is not supported on %s\n", s->name);
> +		return -ENODEV;
> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * check_io_alloc_enabled() - Establish if io_alloc is enabled
> + *
> + * @s: resctrl resource schema
> + *
> + * This function must be called under the cpu hotplug lock
> + * and rdtgroup mutex
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +static int check_io_alloc_enabled(struct resctrl_schema *s)
> +{
> +	struct rdt_resource *r = s->res;
> +
> +	if (!resctrl_arch_get_io_alloc_enabled(r)) {
> +		rdt_last_cmd_printf("io_alloc is not enabled on %s\n", s->name);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>   ssize_t resctrl_io_alloc_write(struct kernfs_open_file *of, char *buf,
>   			       size_t nbytes, loff_t off)
>   {
> @@ -777,11 +821,9 @@ ssize_t resctrl_io_alloc_write(struct kernfs_open_file *of, char *buf,
>   
>   	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;
> +	ret = check_io_alloc_support(s);
> +	if (ret)
>   		goto out_unlock;
> -	}
>   
>   	/* If the feature is already up to date, no action is needed. */
>   	if (resctrl_arch_get_io_alloc_enabled(r) == enable)
> @@ -828,6 +870,47 @@ ssize_t resctrl_io_alloc_write(struct kernfs_open_file *of, char *buf,
>   	return ret ?: nbytes;
>   }
>   
> +ssize_t resctrl_io_alloc_cbm_min_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;
> +	bool reset;
> +	u32 io_alloc_closid;
> +	int ret;
> +
> +	ret = kstrtobool(buf, &reset);
> +	if (ret)
> +		return ret;
> +	if (reset)
> +		return -EINVAL;
> +
> +	cpus_read_lock();
> +	mutex_lock(&rdtgroup_mutex);
> +
> +	rdt_last_cmd_clear();
> +
> +	ret = check_io_alloc_support(s);
> +	if (ret)
> +		goto out_unlock;
> +
> +	ret = check_io_alloc_enabled(s);
> +	if (ret)
> +		goto out_unlock;
> +
> +	r->cache.io_alloc.io_alloc_min_cbm = true;
> +
> +	io_alloc_closid = resctrl_io_alloc_closid(r);
> +	ret = resctrl_arch_update_domains(r, io_alloc_closid);
> +
> +	r->cache.io_alloc.io_alloc_min_cbm = false;
> +out_unlock:
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}
> +
>   int resctrl_io_alloc_cbm_show(struct kernfs_open_file *of, struct seq_file *seq, void *v)
>   {
>   	struct resctrl_schema *s = rdt_kn_parent_priv(of->kn);
> @@ -839,17 +922,13 @@ int resctrl_io_alloc_cbm_show(struct kernfs_open_file *of, struct seq_file *seq,
>   
>   	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;
> +	ret = check_io_alloc_support(s);
> +	if (ret)
>   		goto out_unlock;
> -	}
>   
> -	if (!resctrl_arch_get_io_alloc_enabled(r)) {
> -		rdt_last_cmd_printf("io_alloc is not enabled on %s\n", s->name);
> -		ret = -EINVAL;
> +	ret = check_io_alloc_enabled(s);
> +	if (ret)
>   		goto out_unlock;
> -	}
>   
>   	/*
>   	 * When CDP is enabled, the CBMs of the highest CLOSID of CDP_CODE and
> @@ -928,17 +1007,13 @@ ssize_t resctrl_io_alloc_cbm_write(struct kernfs_open_file *of, char *buf,
>   	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;
> +	ret = check_io_alloc_support(s);
> +	if (ret)
>   		goto out_unlock;
> -	}
>   
> -	if (!resctrl_arch_get_io_alloc_enabled(r)) {
> -		rdt_last_cmd_printf("io_alloc is not enabled on %s\n", s->name);
> -		ret = -EINVAL;
> +	ret = check_io_alloc_enabled(s);
> +	if (ret)
>   		goto out_unlock;
> -	}
>   
>   	io_alloc_closid = resctrl_io_alloc_closid(r);
>   
> diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
> index bff4a54ae333..f50f1ab562b0 100644
> --- a/fs/resctrl/internal.h
> +++ b/fs/resctrl/internal.h
> @@ -442,6 +442,9 @@ int resctrl_io_alloc_cbm_show(struct kernfs_open_file *of, struct seq_file *seq,
>   			      void *v);
>   ssize_t resctrl_io_alloc_cbm_write(struct kernfs_open_file *of, char *buf,
>   				   size_t nbytes, loff_t off);
> +ssize_t resctrl_io_alloc_cbm_min_write(struct kernfs_open_file *of, char *buf,
> +				       size_t nbytes, loff_t off);
> +
>   u32 resctrl_io_alloc_closid(struct rdt_resource *r);
>   
>   #ifdef CONFIG_RESCTRL_FS_PSEUDO_LOCK
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index ea320dcf8aba..bd41ab5a8eb4 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -1995,6 +1995,12 @@ static struct rftype res_common_files[] = {
>   		.seq_show	= resctrl_io_alloc_cbm_show,
>   		.write		= resctrl_io_alloc_cbm_write,
>   	},
> +	{
> +		.name		= "io_alloc_min_cbm_all",
> +		.mode		= 0644,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.write		= resctrl_io_alloc_cbm_min_write,
> +	},
>   	{
>   		.name		= "max_threshold_occupancy",
>   		.mode		= 0644,
> @@ -2195,11 +2201,13 @@ static void io_alloc_init(void)
>   {
>   	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
>   
> -	if (r->cache.io_alloc_capable) {
> +	if (r->cache.io_alloc.io_alloc_capable) {
>   		resctrl_file_fflags_init("io_alloc", RFTYPE_CTRL_INFO |
>   					 RFTYPE_RES_CACHE);
>   		resctrl_file_fflags_init("io_alloc_cbm",
>   					 RFTYPE_CTRL_INFO | RFTYPE_RES_CACHE);
> +		resctrl_file_fflags_init("io_alloc_min_cbm_all",
> +					 RFTYPE_CTRL_INFO | RFTYPE_RES_CACHE);
>   	}
>   }
>   
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 54701668b3df..7987d458ebf8 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -215,7 +215,10 @@ struct resctrl_cache {
>   	unsigned int	shareable_bits;
>   	bool		arch_has_sparse_bitmasks;
>   	bool		arch_has_per_cpu_cfg;
> -	bool		io_alloc_capable;
> +	struct {
> +		bool	io_alloc_capable;
> +		bool	io_alloc_min_cbm;
> +	} io_alloc;
>   };
>   
>   /**
> @@ -415,6 +418,31 @@ static inline bool resctrl_is_mbm_event(enum resctrl_event_id eventid)
>   		eventid <= QOS_L3_MBM_LOCAL_EVENT_ID);
>   }
>   
> +/*
> + * apply_io_alloc_min_cbm() - Apply minimum io_alloc CBM
> + *
> + * @r: resctrl resource
> + *
> + * Return: Minimum number of consecutive io_alloc CBM bits to be set.
> + */
> +static inline u32 apply_io_alloc_min_cbm(struct rdt_resource *r)
> +{
> +	return r->cache.min_cbm_bits;
> +}
> +
> +/*
> + * resctrl_should_io_alloc_min_cbm() - Should the minimum io_alloc
> + *				       CBM be applied
> + * @r: resctrl resource
> + *
> + * Return: True if the minimum number of consecutive
> + * bits to be set in the io_alloc CBM should be applied.
> + */
> +static inline bool resctrl_should_io_alloc_min_cbm(struct rdt_resource *r)
> +{
> +	return r->cache.io_alloc.io_alloc_min_cbm;
> +}
> +
>   u32 resctrl_get_mon_evt_cfg(enum resctrl_event_id eventid);
>   
>   /* Iterate over all memory bandwidth events */