Re: [PATCH v2 02/18] x86/resctrl: Access per-rmid structures by index

[James Morse <james.morse@arm.com>]

Hi Reinette,

On 02/02/2023 23:44, Reinette Chatre wrote:
> On 1/13/2023 9:54 AM, James Morse wrote:
>> Because of the differences between Intel RDT/AMD QoS and Arm's MPAM
>> monitors, RMID values on arm64 are not unique unless the CLOSID is
>> also included. Bitmaps like rmid_busy_llc need to be sized by the
>> number of unique entries for this resource.
>>
>> Add helpers to encode/decode the CLOSID and RMID to an index. The
>> domain's busy_rmid_llc and the rmid_ptrs[] array are then sized by
> 
> busy_rmid_llc -> rmid_busy_llc ?
> 
> Could you please also mention the MBM state impacted?

Yup, this paragraph reworded as:
| Add helpers to encode/decode the CLOSID and RMID to an index. The
| domain's rmid_busy__llc and the rmid_ptrs[] array are then sized by
| index, as are the domain mbm_local and mbm_total arrays.
| On x86, the index is always just the RMID, so all these structures
| remain the same size.
|
| The index gives resctrl a unique value it can use to store monitor
| values, and allows MPAM to decode the closid when reading the hardware
| counters.

>> diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
>> index 52788f79786f..44d568f3577c 100644
>> --- a/arch/x86/include/asm/resctrl.h
>> +++ b/arch/x86/include/asm/resctrl.h
>> @@ -7,6 +7,13 @@
>>  #include <linux/sched.h>
>>  #include <linux/jump_label.h>
>>  
>> +/*
>> + * This value can never be a valid CLOSID, and is used when mapping a
>> + * (closid, rmid) pair to an index and back. On x86 only the RMID is
>> + * needed.
>> + */
>> +#define X86_RESCTRL_BAD_CLOSID		~0

> Should this be moved to previous patch where first usage of ~0 appears?

Makes sense,


> Also, not having a size creates opportunity for inconsistencies. How
> about ((u32)~0) ?

Yes, the compilers secret expectations on sizes always catch me out!


>> +
>>  /**
>>   * struct resctrl_pqr_state - State cache for the PQR MSR
>>   * @cur_rmid:		The cached Resource Monitoring ID
>> @@ -94,6 +101,23 @@ static inline void resctrl_sched_in(void)
>>  		__resctrl_sched_in();
>>  }
>>  
>> +static inline u32 resctrl_arch_system_num_rmid_idx(void)
>> +{
>> +	/* RMID are independent numbers for x86. num_rmid_idx==num_rmid */
>> +	return boot_cpu_data.x86_cache_max_rmid + 1;
>> +}

> It seems that this helper and its subsequent usage eliminates the
> need for struct rdt_resource::num_rmid? Are any users left?

The only user in the filesystem parts of resctrl is rdt_num_rmids_show(), which exposes
the value to user-space. The value is unfortunately meaningless on MPAM systems, but as
its user-space ABI, it has to stay.

The remaining users in the x86 specific code:
domain_add_cpu() continues to use r->num_rmid as the arch code can know its the same
number as num_idx, and rdt_get_mon_l3_config() calculates the value.


Thanks,

James