[PATCH v7 37/49] x86/resctrl: Expand the width of dom_id by replacing mon_data_bits

[Amit Singh Tomar <amitsinght@marvell.com>]

Hi James,

> MPAM platforms retrieve the cache-id property from the ACPI PPTT table.
> The cache-id field is 32 bits wide. Under resctrl, the cache-id becomes
> the domain-id, and is packed into the mon_data_bits union bitfield.
> The width of cache-id in this field is 14 bits.
>
> Expanding the union would break 32bit x86 platforms as this union is
> stored as the kernfs kn->priv pointer. This saved allocating memory
> for the priv data storage.
>
> The firmware on MPAM platforms have used the PPTT cache-id field to
> expose the interconnect's id for the cache, which is sparse and uses
> more than 14 bits. Use of this id is to enable PCIe direct cache
> injection hints. Using this feature with VFIO means the value provided
> by the ACPI table should be exposed to user-space.
>
> To support cache-id values greater than 14 bits, convert the
> mon_data_bits union to a structure. This is allocated for the default
> control group when the kernfs event files are created, and free'd when
> the monitor directory is rmdir'd when the domain goes offline.
> All other control and monitor groups lookup the struct mon_data allocated
> for the default control group, and use this.
> This simplifies the lifecycle of this structure as the default control
> group cannot be rmdir()d by user-space, so only needs to consider
> domain-offline, which removes all the event files corresponding to a
> domain while holding rdtgroup_mutex - which prevents concurrent
> readers. mkdir_mondata_subdir_allrdtgrp() must special case the default
> control group to ensure it is created first.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Previously the MPAM tree repainted the cache-id to compact them,
> argue-ing there was no other user. With VFIO use of this PCIe feature,
> this is no longer an option.
>
> Changes since v6:
>   * Added the get/put helpers.
>   * Special case the creation of the mondata files for the default control
>     group.
>   * Removed wording about files living longer than expected, the corresponding
>     error handling is wrapped in WARN_ON_ONCE() as this indicates a bug.
> ---
>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  19 +--
>   arch/x86/kernel/cpu/resctrl/internal.h    |  37 +++---
>   arch/x86/kernel/cpu/resctrl/monitor.c     |   3 +
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c    | 142 ++++++++++++++++++++--
>   4 files changed, 162 insertions(+), 39 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index 0a0ac5f6112e..159972c3fe73 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -667,7 +667,7 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
>   	u32 resid, evtid, domid;
>   	struct rdtgroup *rdtgrp;
>   	struct rdt_resource *r;
> -	union mon_data_bits md;
> +	struct mon_data *md;
>   	int ret = 0;
>   
>   	rdtgrp = rdtgroup_kn_lock_live(of->kn);
> @@ -676,17 +676,22 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
>   		goto out;
>   	}
>   
> -	md.priv = of->kn->priv;
> -	resid = md.u.rid;
> -	domid = md.u.domid;
> -	evtid = md.u.evtid;
> +	md = of->kn->priv;
> +	if (WARN_ON_ONCE(!md)) {
> +		ret = -EIO;
> +		goto out;
> +	}
> +
> +	resid = md->rid;
> +	domid = md->domid;
> +	evtid = md->evtid;
>   	r = resctrl_arch_get_resource(resid);
>   
> -	if (md.u.sum) {
> +	if (md->sum) {
>   		/*
>   		 * This file requires summing across all domains that share
>   		 * the L3 cache id that was provided in the "domid" field of the
> -		 * mon_data_bits union. Search all domains in the resource for
> +		 * struct mon_data. Search all domains in the resource for
>   		 * one that matches this cache id.
>   		 */
>   		list_for_each_entry(d, &r->mon_domains, hdr.list) {
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 32ed9aeffb90..16c1a391d012 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -103,27 +103,24 @@ struct mon_evt {
>   };
>   
>   /**
> - * union mon_data_bits - Monitoring details for each event file.
> - * @priv:              Used to store monitoring event data in @u
> - *                     as kernfs private data.
> - * @u.rid:             Resource id associated with the event file.
> - * @u.evtid:           Event id associated with the event file.
> - * @u.sum:             Set when event must be summed across multiple
> - *                     domains.
> - * @u.domid:           When @u.sum is zero this is the domain to which
> - *                     the event file belongs. When @sum is one this
> - *                     is the id of the L3 cache that all domains to be
> - *                     summed share.
> - * @u:                 Name of the bit fields struct.
> + * struct mon_data - Monitoring details for each event file.
> + * @rid:             Resource id associated with the event file.
> + * @evtid:           Event id associated with the event file.
> + * @sum:             Set when event must be summed across multiple
> + *                   domains.
> + * @domid:           When @sum is zero this is the domain to which
> + *                   the event file belongs. When @sum is one this
> + *                   is the id of the L3 cache that all domains to be
> + *                   summed share.
> + *
> + * Stored in the kernfs kn->priv field, readers and writers must hold
> + * rdtgroup_mutex.
>    */
> -union mon_data_bits {
> -	void *priv;
> -	struct {
> -		unsigned int rid		: 10;
> -		enum resctrl_event_id evtid	: 7;
> -		unsigned int sum		: 1;
> -		unsigned int domid		: 14;
> -	} u;
> +struct mon_data {
> +	unsigned int rid;
> +	enum resctrl_event_id evtid;
> +	unsigned int sum;
> +	unsigned int domid;
>   };
>   
>   /**
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 73e3fe4f4c87..5bf67b429dd3 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -1096,6 +1096,9 @@ static struct mon_evt mbm_local_event = {
>    * Note that MBM events are also part of RDT_RESOURCE_L3 resource
>    * because as per the SDM the total and local memory bandwidth
>    * are enumerated as part of L3 monitoring.
> + *
> + * mon_put_default_kn_priv_all() also assumes monitor events are only supported
> + * on the L3 resource.
>    */
>   static void l3_mon_evt_init(struct rdt_resource *r)
>   {
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index aecd3fa734cd..443635d195f0 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -3114,6 +3114,110 @@ static struct file_system_type rdt_fs_type = {
>   	.kill_sb		= rdt_kill_sb,
>   };
>   
> +/**
> + * mon_get_default_kn_priv() - Get the mon_data priv data for this event from
> + *                             the default control group.
> + * Called when monitor event files are created for a domain.
> + * When called with the default control group, the structure will be allocated.
> + * This happens at mount time, before other control or monitor groups are
> + * created.
> + * This simplifies the lifetime management for rmdir() versus domain-offline
> + * as the default control group lives forever, and only one group needs to be
> + * special cased.
> + *
> + * @r:      The resource for the event type being created.
> + * @d:	    The domain for the event type being created.
> + * @mevt:   The event type being created.
> + * @rdtgrp: The rdtgroup for which the monitor file is being created,
> + *          used to determine if this is the default control group.
> + * @do_sum: Whether the SNC sub-numa node monitors are being created.
> + */
> +static struct mon_data *mon_get_default_kn_priv(struct rdt_resource *r,
> +						struct rdt_mon_domain *d,
> +						struct mon_evt *mevt,
> +						struct rdtgroup *rdtgrp,
> +						bool do_sum)
> +{
> +	struct kernfs_node *kn_dom, *kn_evt;
> +	struct mon_data *priv;
> +	bool snc_mode;
> +	char name[32];
> +
> +	lockdep_assert_held(&rdtgroup_mutex);
> +
> +	snc_mode = r->mon_scope == RESCTRL_L3_NODE;
> +	if (!do_sum)
> +		sprintf(name, "mon_%s_%02d", r->name, snc_mode ? d->ci->id : d->hdr.id);
This change triggered a minor report during compilation.

fs/resctrl/rdtgroup.c: In function ‘mon_get_default_kn_priv’:
fs/resctrl/rdtgroup.c:2931:28: warning: format ‘%d’ expects argument of 
type ‘int’, but argument 4 has type ‘long unsigned int’ [-Wformat=]
  2931 |   sprintf(name, "mon_%s_%02d", r->name, snc_mode ? d->ci->id : 
d->hdr.id);
       |                         ~~~^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       | |                                 |
       | int                               long unsigned int
       |                         %02ld

> +	else
> +		sprintf(name, "mon_sub_%s_%02d", r->name, d->hdr.id);
> +
> +	kn_dom = kernfs_find_and_get(kn_mondata, name);
> +	if (!kn_dom)
> +		return NULL;
> +
> +	kn_evt = kernfs_find_and_get(kn_dom, mevt->name);
> +
> +	/* Is this the creation of the default groups monitor files? */
> +	if (!kn_evt && rdtgrp == &rdtgroup_default) {
> +		priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> +		if (!priv)
> +			return NULL;
> +		priv->rid = r->rid;
> +		priv->domid = do_sum ? d->ci->id : d->hdr.id;
> +		priv->sum = do_sum;
> +		priv->evtid = mevt->evtid;
> +		return priv;
> +	}
> +
> +	if (!kn_evt)
> +		return NULL;
> +
> +	return kn_evt->priv;
> +}
> +
> +/**
> + * mon_put_default_kn_priv_all() - Potentially free the mon_data priv data for
> + *                                 all events from the default control group.
> + * Put the mon_data priv data for all events for a particular domain.
> + * When called with the default control group, the priv structure previously
> + * allocated will be kfree()d. This should only be done as part of taking a
> + * domain offline.
> + * Only a domain offline will 'rmdir' monitor files in the default control
> + * group. After domain offline releases rdtgrp_mutex, all references will
> + * have been removed.
> + *
> + * @rdtgrp:  The rdtgroup for which the monitor files are being removed,
> + *           used to determine if this is the default control group.
> + * @name:    The name of the domain or SNC sub-numa domain which is being
> + *           taken offline.
> + */
> +static void mon_put_default_kn_priv_all(struct rdtgroup *rdtgrp, char *name)
> +{
> +	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
> +	struct kernfs_node *kn_dom, *kn_evt;
> +	struct mon_evt *mevt;
> +
> +	lockdep_assert_held(&rdtgroup_mutex);
> +
> +	if (rdtgrp != &rdtgroup_default)
> +		return;
> +
> +	kn_dom = kernfs_find_and_get(kn_mondata, name);
> +	if (!kn_dom)
> +		return;
> +
> +	list_for_each_entry(mevt, &r->evt_list, list) {
> +		kn_evt = kernfs_find_and_get(kn_dom, mevt->name);
> +		if (!kn_evt)
> +			continue;
> +		if (!kn_evt->priv)
> +			continue;
> +
> +		kfree(kn_evt->priv);
> +		kn_evt->priv = NULL;
> +	}
> +}
> +
>   static int mon_addfile(struct kernfs_node *parent_kn, const char *name,
>   		       void *priv)
>   {
> @@ -3135,19 +3239,25 @@ static int mon_addfile(struct kernfs_node *parent_kn, const char *name,
>   	return ret;
>   }
>   
> -static void mon_rmdir_one_subdir(struct kernfs_node *pkn, char *name, char *subname)
> +static void mon_rmdir_one_subdir(struct rdtgroup *rdtgrp, char *name, char *subname)
>   {
> +	struct kernfs_node *pkn = rdtgrp->mon.mon_data_kn;
>   	struct kernfs_node *kn;
>   
>   	kn = kernfs_find_and_get(pkn, name);
>   	if (!kn)
>   		return;
> +
> +	mon_put_default_kn_priv_all(rdtgrp, name);
> +
>   	kernfs_put(kn);
>   
> -	if (kn->dir.subdirs <= 1)
> +	if (kn->dir.subdirs <= 1) {
>   		kernfs_remove(kn);
> -	else
> +	} else {
> +		mon_put_default_kn_priv_all(rdtgrp, subname);
>   		kernfs_remove_by_name(kn, subname);
> +	}
>   }
>   
>   /*
> @@ -3170,10 +3280,10 @@ static void rmdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
>   		sprintf(subname, "mon_sub_%s_%02d", r->name, d->hdr.id);
>   
>   	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
> -		mon_rmdir_one_subdir(prgrp->mon.mon_data_kn, name, subname);
> +		mon_rmdir_one_subdir(prgrp, name, subname);
>   
>   		list_for_each_entry(crgrp, &prgrp->mon.crdtgrp_list, mon.crdtgrp_list)
> -			mon_rmdir_one_subdir(crgrp->mon.mon_data_kn, name, subname);
> +			mon_rmdir_one_subdir(crgrp, name, subname);
>   	}
>   }
>   
> @@ -3182,19 +3292,19 @@ static int mon_add_all_files(struct kernfs_node *kn, struct rdt_mon_domain *d,
>   			     bool do_sum)
>   {
>   	struct rmid_read rr = {0};
> -	union mon_data_bits priv;
> +	struct mon_data *priv;
>   	struct mon_evt *mevt;
>   	int ret;
>   
>   	if (WARN_ON(list_empty(&r->evt_list)))
>   		return -EPERM;
>   
> -	priv.u.rid = r->rid;
> -	priv.u.domid = do_sum ? d->ci->id : d->hdr.id;
> -	priv.u.sum = do_sum;
>   	list_for_each_entry(mevt, &r->evt_list, list) {
> -		priv.u.evtid = mevt->evtid;
> -		ret = mon_addfile(kn, mevt->name, priv.priv);
> +		priv = mon_get_default_kn_priv(r, d, mevt, prgrp, do_sum);
> +		if (WARN_ON_ONCE(!priv))
> +			return -EINVAL;
> +
> +		ret = mon_addfile(kn, mevt->name, priv);
>   		if (ret)
>   			return ret;
>   
> @@ -3274,9 +3384,17 @@ static void mkdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
>   	struct rdtgroup *prgrp, *crgrp;
>   	struct list_head *head;
>   
> +	/*
> +	 * During domain-online create the default control group first
> +	 * so that mon_get_default_kn_priv() can find the allocated structure
> +	 * on subsequent calls.
> +	 */
> +	mkdir_mondata_subdir(kn_mondata, d, r, &rdtgroup_default);
> +
>   	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
>   		parent_kn = prgrp->mon.mon_data_kn;
> -		mkdir_mondata_subdir(parent_kn, d, r, prgrp);
> +		if (prgrp != &rdtgroup_default)
> +			mkdir_mondata_subdir(parent_kn, d, r, prgrp);
>   
>   		head = &prgrp->mon.crdtgrp_list;
>   		list_for_each_entry(crgrp, head, mon.crdtgrp_list) {
> -- 
> 2.39.5
>

On 3/1/2025 1:29 AM, James Morse wrote:
> MPAM platforms retrieve the cache-id property from the ACPI PPTT 
> table. The cache-id field is 32 bits wide. Under resctrl, the cache-id 
> becomes the domain-id, and is packed into the mon_data_bits union 
> bitfield. The width of cache-id in this
> 
> MPAM platforms retrieve the cache-id property from the ACPI PPTT table.
> The cache-id field is 32 bits wide. Under resctrl, the cache-id becomes
> the domain-id, and is packed into the mon_data_bits union bitfield.
> The width of cache-id in this field is 14 bits.
>
> Expanding the union would break 32bit x86 platforms as this union is
> stored as the kernfs kn->priv pointer. This saved allocating memory
> for the priv data storage.
>
> The firmware on MPAM platforms have used the PPTT cache-id field to
> expose the interconnect's id for the cache, which is sparse and uses
> more than 14 bits. Use of this id is to enable PCIe direct cache
> injection hints. Using this feature with VFIO means the value provided
> by the ACPI table should be exposed to user-space.
>
> To support cache-id values greater than 14 bits, convert the
> mon_data_bits union to a structure. This is allocated for the default
> control group when the kernfs event files are created, and free'd when
> the monitor directory is rmdir'd when the domain goes offline.
> All other control and monitor groups lookup the struct mon_data allocated
> for the default control group, and use this.
> This simplifies the lifecycle of this structure as the default control
> group cannot be rmdir()d by user-space, so only needs to consider
> domain-offline, which removes all the event files corresponding to a
> domain while holding rdtgroup_mutex - which prevents concurrent
> readers. mkdir_mondata_subdir_allrdtgrp() must special case the default
> control group to ensure it is created first.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Previously the MPAM tree repainted the cache-id to compact them,
> argue-ing there was no other user. With VFIO use of this PCIe feature,
> this is no longer an option.
>
> Changes since v6:
>   * Added the get/put helpers.
>   * Special case the creation of the mondata files for the default control
>     group.
>   * Removed wording about files living longer than expected, the corresponding
>     error handling is wrapped in WARN_ON_ONCE() as this indicates a bug.
> ---
>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  19 +--
>   arch/x86/kernel/cpu/resctrl/internal.h    |  37 +++---
>   arch/x86/kernel/cpu/resctrl/monitor.c     |   3 +
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c    | 142 ++++++++++++++++++++--
>   4 files changed, 162 insertions(+), 39 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index 0a0ac5f6112e..159972c3fe73 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -667,7 +667,7 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
>   	u32 resid, evtid, domid;
>   	struct rdtgroup *rdtgrp;
>   	struct rdt_resource *r;
> -	union mon_data_bits md;
> +	struct mon_data *md;
>   	int ret = 0;
>   
>   	rdtgrp = rdtgroup_kn_lock_live(of->kn);
> @@ -676,17 +676,22 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
>   		goto out;
>   	}
>   
> -	md.priv = of->kn->priv;
> -	resid = md.u.rid;
> -	domid = md.u.domid;
> -	evtid = md.u.evtid;
> +	md = of->kn->priv;
> +	if (WARN_ON_ONCE(!md)) {
> +		ret = -EIO;
> +		goto out;
> +	}
> +
> +	resid = md->rid;
> +	domid = md->domid;
> +	evtid = md->evtid;
>   	r = resctrl_arch_get_resource(resid);
>   
> -	if (md.u.sum) {
> +	if (md->sum) {
>   		/*
>   		 * This file requires summing across all domains that share
>   		 * the L3 cache id that was provided in the "domid" field of the
> -		 * mon_data_bits union. Search all domains in the resource for
> +		 * struct mon_data. Search all domains in the resource for
>   		 * one that matches this cache id.
>   		 */
>   		list_for_each_entry(d, &r->mon_domains, hdr.list) {
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 32ed9aeffb90..16c1a391d012 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -103,27 +103,24 @@ struct mon_evt {
>   };
>   
>   /**
> - * union mon_data_bits - Monitoring details for each event file.
> - * @priv:              Used to store monitoring event data in @u
> - *                     as kernfs private data.
> - * @u.rid:             Resource id associated with the event file.
> - * @u.evtid:           Event id associated with the event file.
> - * @u.sum:             Set when event must be summed across multiple
> - *                     domains.
> - * @u.domid:           When @u.sum is zero this is the domain to which
> - *                     the event file belongs. When @sum is one this
> - *                     is the id of the L3 cache that all domains to be
> - *                     summed share.
> - * @u:                 Name of the bit fields struct.
> + * struct mon_data - Monitoring details for each event file.
> + * @rid:             Resource id associated with the event file.
> + * @evtid:           Event id associated with the event file.
> + * @sum:             Set when event must be summed across multiple
> + *                   domains.
> + * @domid:           When @sum is zero this is the domain to which
> + *                   the event file belongs. When @sum is one this
> + *                   is the id of the L3 cache that all domains to be
> + *                   summed share.
> + *
> + * Stored in the kernfs kn->priv field, readers and writers must hold
> + * rdtgroup_mutex.
>    */
> -union mon_data_bits {
> -	void *priv;
> -	struct {
> -		unsigned int rid		: 10;
> -		enum resctrl_event_id evtid	: 7;
> -		unsigned int sum		: 1;
> -		unsigned int domid		: 14;
> -	} u;
> +struct mon_data {
> +	unsigned int rid;
> +	enum resctrl_event_id evtid;
> +	unsigned int sum;
> +	unsigned int domid;
>   };
>   
>   /**
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 73e3fe4f4c87..5bf67b429dd3 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -1096,6 +1096,9 @@ static struct mon_evt mbm_local_event = {
>    * Note that MBM events are also part of RDT_RESOURCE_L3 resource
>    * because as per the SDM the total and local memory bandwidth
>    * are enumerated as part of L3 monitoring.
> + *
> + * mon_put_default_kn_priv_all() also assumes monitor events are only supported
> + * on the L3 resource.
>    */
>   static void l3_mon_evt_init(struct rdt_resource *r)
>   {
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index aecd3fa734cd..443635d195f0 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -3114,6 +3114,110 @@ static struct file_system_type rdt_fs_type = {
>   	.kill_sb		= rdt_kill_sb,
>   };
>   
> +/**
> + * mon_get_default_kn_priv() - Get the mon_data priv data for this event from
> + *                             the default control group.
> + * Called when monitor event files are created for a domain.
> + * When called with the default control group, the structure will be allocated.
> + * This happens at mount time, before other control or monitor groups are
> + * created.
> + * This simplifies the lifetime management for rmdir() versus domain-offline
> + * as the default control group lives forever, and only one group needs to be
> + * special cased.
> + *
> + * @r:      The resource for the event type being created.
> + * @d:	    The domain for the event type being created.
> + * @mevt:   The event type being created.
> + * @rdtgrp: The rdtgroup for which the monitor file is being created,
> + *          used to determine if this is the default control group.
> + * @do_sum: Whether the SNC sub-numa node monitors are being created.
> + */
> +static struct mon_data *mon_get_default_kn_priv(struct rdt_resource *r,
> +						struct rdt_mon_domain *d,
> +						struct mon_evt *mevt,
> +						struct rdtgroup *rdtgrp,
> +						bool do_sum)
> +{
> +	struct kernfs_node *kn_dom, *kn_evt;
> +	struct mon_data *priv;
> +	bool snc_mode;
> +	char name[32];
> +
> +	lockdep_assert_held(&rdtgroup_mutex);
> +
> +	snc_mode = r->mon_scope == RESCTRL_L3_NODE;
> +	if (!do_sum)
> +		sprintf(name, "mon_%s_%02d", r->name, snc_mode ? d->ci->id : d->hdr.id);
> +	else
> +		sprintf(name, "mon_sub_%s_%02d", r->name, d->hdr.id);
> +
> +	kn_dom = kernfs_find_and_get(kn_mondata, name);
> +	if (!kn_dom)
> +		return NULL;
> +
> +	kn_evt = kernfs_find_and_get(kn_dom, mevt->name);
> +
> +	/* Is this the creation of the default groups monitor files? */
> +	if (!kn_evt && rdtgrp == &rdtgroup_default) {
> +		priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> +		if (!priv)
> +			return NULL;
> +		priv->rid = r->rid;
> +		priv->domid = do_sum ? d->ci->id : d->hdr.id;
> +		priv->sum = do_sum;
> +		priv->evtid = mevt->evtid;
> +		return priv;
> +	}
> +
> +	if (!kn_evt)
> +		return NULL;
> +
> +	return kn_evt->priv;
> +}
> +
> +/**
> + * mon_put_default_kn_priv_all() - Potentially free the mon_data priv data for
> + *                                 all events from the default control group.
> + * Put the mon_data priv data for all events for a particular domain.
> + * When called with the default control group, the priv structure previously
> + * allocated will be kfree()d. This should only be done as part of taking a
> + * domain offline.
> + * Only a domain offline will 'rmdir' monitor files in the default control
> + * group. After domain offline releases rdtgrp_mutex, all references will
> + * have been removed.
> + *
> + * @rdtgrp:  The rdtgroup for which the monitor files are being removed,
> + *           used to determine if this is the default control group.
> + * @name:    The name of the domain or SNC sub-numa domain which is being
> + *           taken offline.
> + */
> +static void mon_put_default_kn_priv_all(struct rdtgroup *rdtgrp, char *name)
> +{
> +	struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);
> +	struct kernfs_node *kn_dom, *kn_evt;
> +	struct mon_evt *mevt;
> +
> +	lockdep_assert_held(&rdtgroup_mutex);
> +
> +	if (rdtgrp != &rdtgroup_default)
> +		return;
> +
> +	kn_dom = kernfs_find_and_get(kn_mondata, name);
> +	if (!kn_dom)
> +		return;
> +
> +	list_for_each_entry(mevt, &r->evt_list, list) {
> +		kn_evt = kernfs_find_and_get(kn_dom, mevt->name);
> +		if (!kn_evt)
> +			continue;
> +		if (!kn_evt->priv)
> +			continue;
> +
> +		kfree(kn_evt->priv);
> +		kn_evt->priv = NULL;
> +	}
> +}
> +
>   static int mon_addfile(struct kernfs_node *parent_kn, const char *name,
>   		       void *priv)
>   {
> @@ -3135,19 +3239,25 @@ static int mon_addfile(struct kernfs_node *parent_kn, const char *name,
>   	return ret;
>   }
>   
> -static void mon_rmdir_one_subdir(struct kernfs_node *pkn, char *name, char *subname)
> +static void mon_rmdir_one_subdir(struct rdtgroup *rdtgrp, char *name, char *subname)
>   {
> +	struct kernfs_node *pkn = rdtgrp->mon.mon_data_kn;
>   	struct kernfs_node *kn;
>   
>   	kn = kernfs_find_and_get(pkn, name);
>   	if (!kn)
>   		return;
> +
> +	mon_put_default_kn_priv_all(rdtgrp, name);
> +
>   	kernfs_put(kn);
>   
> -	if (kn->dir.subdirs <= 1)
> +	if (kn->dir.subdirs <= 1) {
>   		kernfs_remove(kn);
> -	else
> +	} else {
> +		mon_put_default_kn_priv_all(rdtgrp, subname);
>   		kernfs_remove_by_name(kn, subname);
> +	}
>   }
>   
>   /*
> @@ -3170,10 +3280,10 @@ static void rmdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
>   		sprintf(subname, "mon_sub_%s_%02d", r->name, d->hdr.id);
>   
>   	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
> -		mon_rmdir_one_subdir(prgrp->mon.mon_data_kn, name, subname);
> +		mon_rmdir_one_subdir(prgrp, name, subname);
>   
>   		list_for_each_entry(crgrp, &prgrp->mon.crdtgrp_list, mon.crdtgrp_list)
> -			mon_rmdir_one_subdir(crgrp->mon.mon_data_kn, name, subname);
> +			mon_rmdir_one_subdir(crgrp, name, subname);
>   	}
>   }
>   
> @@ -3182,19 +3292,19 @@ static int mon_add_all_files(struct kernfs_node *kn, struct rdt_mon_domain *d,
>   			     bool do_sum)
>   {
>   	struct rmid_read rr = {0};
> -	union mon_data_bits priv;
> +	struct mon_data *priv;
>   	struct mon_evt *mevt;
>   	int ret;
>   
>   	if (WARN_ON(list_empty(&r->evt_list)))
>   		return -EPERM;
>   
> -	priv.u.rid = r->rid;
> -	priv.u.domid = do_sum ? d->ci->id : d->hdr.id;
> -	priv.u.sum = do_sum;
>   	list_for_each_entry(mevt, &r->evt_list, list) {
> -		priv.u.evtid = mevt->evtid;
> -		ret = mon_addfile(kn, mevt->name, priv.priv);
> +		priv = mon_get_default_kn_priv(r, d, mevt, prgrp, do_sum);
> +		if (WARN_ON_ONCE(!priv))
> +			return -EINVAL;
> +
> +		ret = mon_addfile(kn, mevt->name, priv);
>   		if (ret)
>   			return ret;
>   
> @@ -3274,9 +3384,17 @@ static void mkdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
>   	struct rdtgroup *prgrp, *crgrp;
>   	struct list_head *head;
>   
> +	/*
> +	 * During domain-online create the default control group first
> +	 * so that mon_get_default_kn_priv() can find the allocated structure
> +	 * on subsequent calls.
> +	 */
> +	mkdir_mondata_subdir(kn_mondata, d, r, &rdtgroup_default);
> +
>   	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
>   		parent_kn = prgrp->mon.mon_data_kn;
> -		mkdir_mondata_subdir(parent_kn, d, r, prgrp);
> +		if (prgrp != &rdtgroup_default)
> +			mkdir_mondata_subdir(parent_kn, d, r, prgrp);
>   
>   		head = &prgrp->mon.crdtgrp_list;
>   		list_for_each_entry(crgrp, head, mon.crdtgrp_list) {
> -- 
> 2.39.5
>