Re: [PATCH v5 05/24] x86/resctrl: Allow RMID allocation to be scoped by CLOSID

[Fenghua Yu]

Hi, James,

On 7/28/23 09:42, James Morse wrote:
> MPAMs RMID values are not unique unless the CLOSID is considered as well.
>
> alloc_rmid() expects the RMID to be an independent number.
>
> Pass the CLOSID in to alloc_rmid(). Use this to compare indexes when
> allocating. If the CLOSID is not relevant to the index, this ends up
> comparing the free RMID with itself, and the first free entry will be
> used. With MPAM the CLOSID is included in the index, so this becomes a
> walk of the free RMID entries, until one that matches the supplied
> CLOSID is found.
>
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> ---
> Changes since v2;
>   * Rephrased comment in resctrl_find_free_rmid() to describe this in terms of
>     list_entry_first()
>   * Rephrased comment above alloc_rmid()
>
> Changes since v3:
>   * Flipped conditions in alloc_rmid()
>
> Changes since v4:
>   * Typo in comment
> ---
>   arch/x86/kernel/cpu/resctrl/internal.h    |  2 +-
>   arch/x86/kernel/cpu/resctrl/monitor.c     | 51 +++++++++++++++++------
>   arch/x86/kernel/cpu/resctrl/pseudo_lock.c |  2 +-
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c    |  2 +-
>   4 files changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index b48715bb8762..94749ee950dd 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -535,7 +535,7 @@ void rdtgroup_pseudo_lock_remove(struct rdtgroup *rdtgrp);
>   struct rdt_domain *get_domain_from_cpu(int cpu, struct rdt_resource *r);
>   int closids_supported(void);
>   void closid_free(int closid);
> -int alloc_rmid(void);
> +int alloc_rmid(u32 closid);
>   void free_rmid(u32 closid, u32 rmid);
>   int rdt_get_mon_l3_config(struct rdt_resource *r);
>   bool __init rdt_cpu_has(int flag);
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index bd234b66dddf..de91ca781d9f 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -337,24 +337,49 @@ bool has_busy_rmid(struct rdt_domain *d)
>   	return find_first_bit(d->rmid_busy_llc, idx_limit) != idx_limit;
>   }
>   
> -/*
> - * As of now the RMIDs allocation is global.
> - * However we keep track of which packages the RMIDs
> - * are used to optimize the limbo list management.
> - */
> -int alloc_rmid(void)
> +static struct rmid_entry *resctrl_find_free_rmid(u32 closid)
>   {
> -	struct rmid_entry *entry;
> -
> -	lockdep_assert_held(&rdtgroup_mutex);
> +	struct rmid_entry *itr;
> +	u32 itr_idx, cmp_idx;
>   
>   	if (list_empty(&rmid_free_lru))
> -		return rmid_limbo_count ? -EBUSY : -ENOSPC;
> +		return rmid_limbo_count ? ERR_PTR(-EBUSY) : ERR_PTR(-ENOSPC);
> +
> +	list_for_each_entry(itr, &rmid_free_lru, list) {
> +		/*
> +		 * Get the index of this free RMID, and the index it would need
> +		 * to be if it were used with this CLOSID.
> +		 * If the CLOSID is irrelevant on this architecture, these will
> +		 * always be the same meaning the compiler can reduce this loop
> +		 * to a single list_entry_first() call.

s/list_entry_first()/list_first_entry()/?

Seems the comment is not accurate because the loop is not really reduced 
to a single list_first_entry() call. Getting itr_idx and cmp_idx and 
comparing them are extra operations that list_first_entry() doesn't have.

Maybe change the second half comment to something like:

If the CLOSID is irrelevant on this architecture, the two index values 
are always same on every entry and thus the very first entry will be 
returned.

> +		 */
> +		itr_idx = resctrl_arch_rmid_idx_encode(itr->closid, itr->rmid);
> +		cmp_idx = resctrl_arch_rmid_idx_encode(closid, itr->rmid);
> +
> +		if (itr_idx == cmp_idx)
> +			return itr;
> +	}
> +
> +	return ERR_PTR(-ENOSPC);
> +}
> +
> +/*
> + * For MPAM the RMID value is not unique, and has to be considered with
> + * the CLOSID. The (CLOSID, RMID) pair is allocated on all domains, which
> + * allows all domains to be managed by a single limbo list.
> + * Each domain also has a rmid_busy_llc to reduce the work of the limbo handler.
> + */
> +int alloc_rmid(u32 closid)
> +{
> +	struct rmid_entry *entry;
> +
> +	lockdep_assert_held(&rdtgroup_mutex);
> +
> +	entry = resctrl_find_free_rmid(closid);
> +	if (IS_ERR(entry))
> +		return PTR_ERR(entry);
>   
> -	entry = list_first_entry(&rmid_free_lru,
> -				 struct rmid_entry, list);
>   	list_del(&entry->list);
> -
>   	return entry->rmid;
>   }
>   
> diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> index aeadaeb5df9a..5ebd6e54c7f2 100644
> --- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> +++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> @@ -763,7 +763,7 @@ int rdtgroup_locksetup_exit(struct rdtgroup *rdtgrp)
>   	int ret;
>   
>   	if (rdt_mon_capable) {
> -		ret = alloc_rmid();
> +		ret = alloc_rmid(rdtgrp->closid);
>   		if (ret < 0) {
>   			rdt_last_cmd_puts("Out of RMIDs\n");
>   			return ret;
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 7c5cfb373d03..b97e119dbe46 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -3172,7 +3172,7 @@ static int mkdir_rdt_prepare_rmid_alloc(struct rdtgroup *rdtgrp)
>   	if (!rdt_mon_capable)
>   		return 0;
>   
> -	ret = alloc_rmid();
> +	ret = alloc_rmid(rdtgrp->closid);
>   	if (ret < 0) {
>   		rdt_last_cmd_puts("Out of RMIDs\n");
>   		return ret;

Thanks.

-Fenghua