Re: [PATCH v4 19/21] x86/resctrl: Rename and change the units of resctrl_cqm_threshold

[Fenghua Yu <fenghua.yu@intel.com>]

Hi, James,

On Tue, Apr 12, 2022 at 12:44:17PM +0000, James Morse wrote:
> resctrl_cqm_threshold is stored in a hardware specific chunk size,
> but exposed to user-space as bytes.
> 
> This means the filesystem parts of resctrl need to know how the hardware
> counts, to convert the user provided byte value to chunks. The interface
> between the architecture's resctrl code and the filesystem ought to
> treat everything as bytes.
> 
> Change the unit of resctrl_cqm_threshold to bytes. resctrl_arch_rmid_read()
> still returns its value in chunks, so this needs converting to bytes.
> As all the users have been touched, rename the variable to
> resctrl_rmid_realloc_threshold, which describes what the value is for.
> 
> Neither r->num_rmid nor hw_res->mon_scale are guaranteed to be a power
> of 2, so the existing code introduces a rounding error from resctrl's
> theoretical fraction of the cache usage. This behaviour is kept as it
> ensures the user visible value matches the value read from hardware
> when the rmid will be reallocated.
> 
> Reviewed-by: Jamie Iles <quic_jiles@quicinc.com>
> Tested-by: Xin Hao <xhao@linux.alibaba.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Tested-by: Cristian Marussi <cristian.marussi@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v3:
>  * Preserved the rounding errors.
> ---
>  arch/x86/kernel/cpu/resctrl/internal.h |  1 -
>  arch/x86/kernel/cpu/resctrl/monitor.c  | 41 +++++++++++++++-----------
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c |  9 ++----
>  include/linux/resctrl.h                |  2 ++
>  4 files changed, 28 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index bdb55c2fbdd3..c05e9b7cf77a 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -98,7 +98,6 @@ struct rmid_read {
>  	u64			val;
>  };
>  
> -extern unsigned int resctrl_cqm_threshold;
>  extern bool rdt_alloc_capable;
>  extern bool rdt_mon_capable;
>  extern unsigned int rdt_mon_features;
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 88988de0c96c..00f6e27e4e0d 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -37,8 +37,8 @@ static LIST_HEAD(rmid_free_lru);
>   * @rmid_limbo_count     count of currently unused but (potentially)
>   *     dirty RMIDs.
>   *     This counts RMIDs that no one is currently using but that
> - *     may have a occupancy value > intel_cqm_threshold. User can change
> - *     the threshold occupancy value.
> + *     may have a occupancy value > resctrl_rmid_realloc_threshold. User can
> + *     change the threshold occupancy value.
>   */
>  static unsigned int rmid_limbo_count;
>  
> @@ -59,10 +59,10 @@ bool rdt_mon_capable;
>  unsigned int rdt_mon_features;
>  
>  /*
> - * This is the threshold cache occupancy at which we will consider an
> + * This is the threshold cache occupancy in bytes at which we will consider an
>   * RMID available for re-allocation.
>   */
> -unsigned int resctrl_cqm_threshold;
> +unsigned int resctrl_rmid_realloc_threshold;
>  
>  #define CF(cf)	((unsigned long)(1048576 * (cf) + 0.5))
>  
> @@ -223,14 +223,13 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>   */
>  void __check_limbo(struct rdt_domain *d, bool force_free)
>  {
> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> +	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>  	struct rmid_entry *entry;
> -	struct rdt_resource *r;
>  	u32 crmid = 1, nrmid;
>  	bool rmid_dirty;
>  	u64 val = 0;
>  
> -	r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> -
>  	/*
>  	 * Skip RMID 0 and start from RMID 1 and check all the RMIDs that
>  	 * are marked as busy for occupancy < threshold. If the occupancy
> @@ -245,10 +244,12 @@ void __check_limbo(struct rdt_domain *d, bool force_free)
>  		entry = __rmid_entry(nrmid);
>  
>  		if (resctrl_arch_rmid_read(r, d, entry->rmid,
> -					   QOS_L3_OCCUP_EVENT_ID, &val))
> +					   QOS_L3_OCCUP_EVENT_ID, &val)) {
>  			rmid_dirty = true;
> -		else
> -			rmid_dirty = (val >= resctrl_cqm_threshold);
> +		} else {
> +			val *= hw_res->mon_scale;
> +			rmid_dirty = (val >= resctrl_rmid_realloc_threshold);
> +		}
>  
>  		if (force_free || !rmid_dirty) {
>  			clear_bit(entry->rmid, d->rmid_busy_llc);
> @@ -289,13 +290,12 @@ int alloc_rmid(void)
>  
>  static void add_rmid_to_limbo(struct rmid_entry *entry)
>  {
> -	struct rdt_resource *r;
> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> +	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>  	struct rdt_domain *d;
>  	int cpu, err;
>  	u64 val = 0;
>  
> -	r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> -
>  	entry->busy = 0;
>  	cpu = get_cpu();
>  	list_for_each_entry(d, &r->domains, list) {
> @@ -303,7 +303,8 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>  			err = resctrl_arch_rmid_read(r, d, entry->rmid,
>  						     QOS_L3_OCCUP_EVENT_ID,
>  						     &val);
> -			if (err || val <= resctrl_cqm_threshold)
> +			val *= hw_res->mon_scale;
> +			if (err || val <= resctrl_rmid_realloc_threshold)
>  				continue;
>  		}
>  
> @@ -744,6 +745,7 @@ int rdt_get_mon_l3_config(struct rdt_resource *r)
>  	unsigned int mbm_offset = boot_cpu_data.x86_cache_mbm_width_offset;
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>  	unsigned int cl_size = boot_cpu_data.x86_cache_size;
> +	unsigned int threshold;
>  	int ret;
>  
>  	hw_res->mon_scale = boot_cpu_data.x86_cache_occ_scale;
> @@ -762,10 +764,15 @@ int rdt_get_mon_l3_config(struct rdt_resource *r)
>  	 *
>  	 * For a 35MB LLC and 56 RMIDs, this is ~1.8% of the LLC.
>  	 */
> -	resctrl_cqm_threshold = cl_size * 1024 / r->num_rmid;
> +	threshold = cl_size * 1024 / r->num_rmid;
>  
> -	/* h/w works in units of "boot_cpu_data.x86_cache_occ_scale" */

Could you please keep this comment? This comment is still helpful and
meaningful in the context.

> -	resctrl_cqm_threshold /= hw_res->mon_scale;
> +	/*
> +	 * Because num_rmid may not be a power of two, round the value
> +	 * to the nearest multiple of hw_res->mon_scale so it matches a
> +	 * value the hardware will measure. mon_scale may not be a power of 2.
> +	 */
> +	threshold /= hw_res->mon_scale;
> +	resctrl_rmid_realloc_threshold = threshold * hw_res->mon_scale;
>  
>  	ret = dom_data_init(r);
>  	if (ret)
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index f494ca6b8bdd..7c35561e5216 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1030,10 +1030,7 @@ static int rdt_delay_linear_show(struct kernfs_open_file *of,
>  static int max_threshold_occ_show(struct kernfs_open_file *of,
>  				  struct seq_file *seq, void *v)
>  {
> -	struct rdt_resource *r = of->kn->parent->priv;
> -	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> -
> -	seq_printf(seq, "%u\n", resctrl_cqm_threshold * hw_res->mon_scale);
> +	seq_printf(seq, "%u\n", resctrl_rmid_realloc_threshold);
>  
>  	return 0;
>  }
> @@ -1055,7 +1052,6 @@ static int rdt_thread_throttle_mode_show(struct kernfs_open_file *of,
>  static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
>  				       char *buf, size_t nbytes, loff_t off)
>  {
> -	struct rdt_hw_resource *hw_res;
>  	unsigned int bytes;
>  	int ret;
>  
> @@ -1066,8 +1062,7 @@ static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
>  	if (bytes > (boot_cpu_data.x86_cache_size * 1024))
>  		return -EINVAL;
>  
> -	hw_res = resctrl_to_arch_res(of->kn->parent->priv);
> -	resctrl_cqm_threshold = bytes / hw_res->mon_scale;
> +	resctrl_rmid_realloc_threshold = bytes;

Shouldn't bytes be multiples of hw_res->mon_scale? If user inputs non-multiples
value, resctrl_rmid_realloc_threshold will keep the value in the kernel. Is that
right?

But if you convert the input into multiples, user may see a different value when
read it.

Does this argument override the reason why this patch is needed?

>  
>  	return nbytes;
>  }
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index cc6311a40582..fc4805901f1a 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -250,4 +250,6 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>  void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d,
>  			     u32 rmid, enum resctrl_event_id eventid);
>  
> +extern unsigned int resctrl_rmid_realloc_threshold;
> +
>  #endif /* _RESCTRL_H */
> -- 
> 2.30.2
> 

Thanks.

-Fenghua