Re: [PATCH v20 12/18] x86/resctrl: Create Sub-NUMA Cluster (SNC) monitor files

Reinette Chatre
2024-06-20T21:30:08Z

Hi Tony,

On 6/10/24 11:35 AM, Tony Luck wrote:
> When SNC mode is enabled, create subdirectories and files to monitor
> at the SNC node granularity. Legacy behavior is preserved by tagging
> the monitor files at the L3 granularity with the "sum" attribute.
> When the user reads these files the kernel will read monitor data
> from all SNC nodes that share the same L3 cache instance and return
> the aggregated value to the user.
>
> Note that the "domid" field for files that must sum across SNC domains
> has the L3 cache instance id, while non-summing files use the domain id.
>
> The "sum" files do not need to make a call to mon_event_read() to
> initialize the MBM counters. This will be handled by initializing the
> individual SNC nodes that share the L3.
>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
> arch/x86/kernel/cpu/resctrl/rdtgroup.c | 62 +++++++++++++++++++-------
> 1 file changed, 46 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 66acbad1c585..fc7f3f139800 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -3022,7 +3022,8 @@ static void rmdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
> }
>
> static int mon_add_all_files(struct kernfs_node *kn, struct rdt_mon_domain *d,
> - struct rdt_resource *r, struct rdtgroup *prgrp)
> + struct rdt_resource *r, struct rdtgroup *prgrp,
> + bool do_sum)
> {
> union mon_data_bits priv;
> struct mon_evt *mevt;
> @@ -3033,14 +3034,15 @@ static int mon_add_all_files(struct kernfs_node *kn, struct rdt_mon_domain *d,
> return -EPERM;
>
> priv.u.rid = r->rid;
> - priv.u.domid = d->hdr.id;
> + 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);
> if (ret)
> return ret;
>
> - if (is_mbm_event(mevt->evtid))
> + if (!do_sum && is_mbm_event(mevt->evtid))
> mon_event_read(&rr, r, d, prgrp, mevt->evtid, true);
> }
>
> @@ -3051,23 +3053,51 @@ static int mkdir_mondata_subdir(struct kernfs_node *parent_kn,
> struct rdt_mon_domain *d,
> struct rdt_resource *r, struct rdtgroup *prgrp)
> {
> - struct kernfs_node *kn;
> + struct kernfs_node *kn, *ckn;
> char name[32];
> - int ret;
> + bool snc_mode;
> + int ret = 0;
>
> - sprintf(name, "mon_%s_%02d", r->name, d->hdr.id);
> - /* create the directory */
> - kn = kernfs_create_dir(parent_kn, name, parent_kn->mode, prgrp);
> - if (IS_ERR(kn))
> - return PTR_ERR(kn);
> + lockdep_assert_held(&rdtgroup_mutex);
>
> - ret = rdtgroup_kn_set_ugid(kn);
> - if (ret)
> - goto out_destroy;
> + snc_mode = r->mon_scope != RESCTRL_L3_CACHE;

I think that testing that it _should_ be of particular scope will be
easier to understand than testing what it should not be. Can this instead
be a positive check of:
snc_mode = r->mon_scope == RESCTRL_L3_NODE;

> + sprintf(name, "mon_%s_%02d", r->name, d->ci->id);

I find this to be too subtle and potentially confusing since it uses
d->ci->id interchangeable for both SNC and non-SNC mode. I understand
that in non-SNC mode the domain id will be the same as the cache id
but I would prefer that the code use the data structures as intended
instead of relying backdoor on assumptions. Something like:

sprintf(name, "mon_%s_%02d", r->name, snc_mode ? d->ci->id : d->hdr.id);

> + kn = kernfs_find_and_get(parent_kn, name);
> + if (kn) {
> + /*
> + * rdtgroup_mutex will prevent this directory from being
> + * removed. No need to keep this hold.
> + */
> + kernfs_put(kn);
> + } else {
> + kn = kernfs_create_dir(parent_kn, name, parent_kn->mode, prgrp);
> + if (IS_ERR(kn))
> + return PTR_ERR(kn);
>
> - ret = mon_add_all_files(kn, d, r, prgrp);
> - if (ret)
> - goto out_destroy;
> + ret = rdtgroup_kn_set_ugid(kn);
> + if (ret)
> + goto out_destroy;
> + ret = mon_add_all_files(kn, d, r, prgrp, snc_mode);
> + if (ret)
> + goto out_destroy;
> + }
> +
> + if (snc_mode) {
> + sprintf(name, "mon_sub_%s_%02d", r->name, d->hdr.id);
> + ckn = kernfs_create_dir(kn, name, parent_kn->mode, prgrp);
> + if (IS_ERR(ckn)) {
> + ret = -EINVAL;
> + goto out_destroy;
> + }
> +
> + ret = rdtgroup_kn_set_ugid(ckn);
> + if (ret)
> + goto out_destroy;
> +
> + ret = mon_add_all_files(ckn, d, r, prgrp, false);
> + if (ret)
> + goto out_destroy;
> + }
>
> kernfs_activate(kn);
> return 0;

Reinette