Re: [PATCH v3 11/19] x86/resctrl: Allow arch to allocate memory needed in resctrl_arch_rmid_read()

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 4/27/2023 7:19 AM, James Morse wrote:
> On 01/04/2023 00:27, Reinette Chatre wrote:
>> On 3/20/2023 10:26 AM, James Morse wrote:

...

>>>  #include <linux/module.h>
>>>  #include <linux/sizes.h>
>>>  #include <linux/slab.h>
>>> @@ -271,7 +272,7 @@ static void smp_call_rmid_read(void *_arg)
>>>  
>>>  int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>>>  			   u32 closid, u32 rmid, enum resctrl_event_id eventid,
>>> -			   u64 *val)
>>> +			   u64 *val, int ignored)
>>>  {
>>>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>>>  	struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
>>> @@ -317,9 +318,14 @@ void __check_limbo(struct rdt_domain *d, bool force_free)
>>>  	u32 idx_limit = resctrl_arch_system_num_rmid_idx();
>>>  	struct rmid_entry *entry;
>>>  	u32 idx, cur_idx = 1;
>>> +	int arch_mon_ctx;
>>>  	bool rmid_dirty;
>>>  	u64 val = 0;
>>>  
>>> +	arch_mon_ctx = resctrl_arch_mon_ctx_alloc(r, QOS_L3_OCCUP_EVENT_ID);
>>> +	if (arch_mon_ctx < 0)
>>> +		return;
> 
>> The vision for this is not clear to me. When I read that context needs to be allocated
>> I expect it to return a pointer to some new context, not an int. What would the
>> "context" consist of?
> 
> It might just need a different name.
> 
> For MPAM, this is allocating a monitor, which is the hardware that does the counting in
> the cache or the memory controller. The number of monitors is an implementation choice,
> and may not match the number of CLOSID/RMID that are in use. There aren't guaranteed to be
> enough to allocate one for every control or monitor group up front.
> 
> The int being returned is the allocated monitor's index. It identifies which monitor needs
> programming to read the provided CLOSID/RMID, and the counter register to read with the value.

I see.

> 
> I can allocate memory for an int if you think that is clearer.
> (I was hoping to leave that for whoever needs something bigger than a pointer)

I'd rather not complicate it in this way.

>>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>>> index ff7452f644e4..03e4f41cd336 100644
>>> --- a/include/linux/resctrl.h
>>> +++ b/include/linux/resctrl.h
>>> @@ -233,6 +233,7 @@ void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d);
>>>   * @rmid:		rmid of the counter to read.
>>>   * @eventid:		eventid to read, e.g. L3 occupancy.
>>>   * @val:		result of the counter read in bytes.
>>> + * @arch_mon_ctx:	An allocated context from resctrl_arch_mon_ctx_alloc().
>>>   *
> 
>> Could this description be expanded to indicate what this context is used for?
> 
> Sure,
> "An architecture specific value from resctrl_arch_mon_ctx_alloc(), for MPAM this
> identifies the hardware monitor allocated for this read request".

This helps. Thank you.

Reinette