Message-ID: <bc26b599-0a0e-46f0-bfda-83330a34293e@amd.com>
Date: Mon, 2 Dec 2024 15:28:27 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Peter Newman <peternewman@...gle.com>
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Subject: Re: [PATCH v9 14/26] x86/resctrl: Introduce interface to display
 number of free counters

Hi Reinette,

On 12/2/24 15:09, Reinette Chatre wrote:
> Hi Babu,
> 
> On 12/2/24 12:42 PM, Moger, Babu wrote:
>> Hi Reinette,
>>
>> On 12/2/24 14:15, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 12/2/24 11:48 AM, Moger, Babu wrote:
>>>> On 12/2/24 12:33, Reinette Chatre wrote:
>>>>> On 11/29/24 9:06 AM, Moger, Babu wrote:
>>>>>> On 11/29/2024 3:59 AM, Peter Newman wrote:
>>>>>>> On Thu, Nov 28, 2024 at 8:35 PM Moger, Babu <bmoger@....com> wrote:
>>>>>>>> On 11/28/2024 5:10 AM, Peter Newman wrote:
>>>>>>>>> On Wed, Nov 27, 2024 at 8:05 PM Reinette Chatre
>>>>>>>>> <reinette.chatre@...el.com> wrote:
>>>>>>>>>>
>>>>>>>>>> Hi Babu,
>>>>>>>>>>
>>>>>>>>>> On 11/27/24 6:57 AM, Moger, Babu wrote:
>>>>>>>
>>>>>>>>>>> 1. Each group needs to remember counter ids in each domain for each event.
>>>>>>>>>>>      For example:
>>>>>>>>>>>      Resctrl group mon1
>>>>>>>>>>>       Total event
>>>>>>>>>>>       dom 0 cntr_id 1,
>>>>>>>>>>>       dom 1 cntr_id 10
>>>>>>>>>>>       dom 2 cntr_id 11
>>>>>>>>>>>
>>>>>>>>>>>      Local event
>>>>>>>>>>>       dom 0 cntr_id 2,
>>>>>>>>>>>       dom 1 cntr_id 15
>>>>>>>>>>>       dom 2 cntr_id 10
>>>>>>>>>>
>>>>>>>>>> Indeed. The challenge here is that domains may come and go so it cannot be a simple
>>>>>>>>>> static array. As an alternative it can be an xarray indexed by the domain ID with
>>>>>>>>>> pointers to a struct like below to contain the counters associated with the monitor
>>>>>>>>>> group:
>>>>>>>>>>           struct cntr_id {
>>>>>>>>>>                   u32     mbm_total;
>>>>>>>>>>                   u32     mbm_local;
>>>>>>>>>>           }
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Thinking more about how this array needs to be managed made me wonder how the
>>>>>>>>>> current implementation deals with domains that come and go. I do not think
>>>>>>>>>> this is currently handled. For example, if a new domain comes online and
>>>>>>>>>> monitoring groups had counters dynamically assigned, then these counters are
>>>>>>>>>> not configured to the newly online domain.
>>>>>>>>
>>>>>>>> I am trying to understand the details of your approach here.
>>>>>>>>>
>>>>>>>>> In my prototype, I allocated a counter id-indexed array to each
>>>>>>>>> monitoring domain structure for tracking the counter allocations,
>>>>>>>>> because the hardware counters are all domain-scoped. That way the
>>>>>>>>> tracking data goes away when the hardware does.
>>>>>>>>>
>>>>>>>>> I was focused on allowing all pending counter updates to a domain
>>>>>>>>> resulting from a single mbm_assign_control write to be batched and
>>>>>>>>> processed in a single IPI, so I structured the counter tracker
>>>>>>>>> something like this:
>>>>>>>>
>>>>>>>> Not sure what you meant here. How are you batching two IPIs for two domains?
>>>>>>>>
>>>>>>>> #echo "//0=t;1=t" > /sys/fs/resctrl/info/L3_MON/mbm_assign_control
>>>>>>>>
>>>>>>>> This is still a single write. Two IPIs are sent separately, one for each
>>>>>>>> domain.
>>>>>>>>
>>>>>>>> Are you doing something different?
>>>>>>>
>>>>>>> I said "all pending counter updates to a domain", whereby I meant
>>>>>>> targeting a single domain.
>>>>>>>
>>>>>>> Depending on the CPU of the caller, your example write requires 1 or 2 IPIs.
>>>>>>>
>>>>>>> What is important is that the following write also requires 1 or 2 IPIs:
>>>>>>>
>>>>>>> (assuming /sys/fs/resctrl/mon_groups/[g1-g31] exist, line breaks added
>>>>>>> for readability)
>>>>>>>
>>>>>>> echo $'//0=t;1=t\n
>>>>>>> /g1/0=t;1=t\n
>>>>>>> /g2/0=t;1=t\n
>>>>>>> /g3/0=t;1=t\n
>>>>>>> /g4/0=t;1=t\n
>>>>>>> /g5/0=t;1=t\n
>>>>>>> /g6/0=t;1=t\n
>>>>>>> /g7/0=t;1=t\n
>>>>>>> /g8/0=t;1=t\n
>>>>>>> /g9/0=t;1=t\n
>>>>>>> /g10/0=t;1=t\n
>>>>>>> /g11/0=t;1=t\n
>>>>>>> /g12/0=t;1=t\n
>>>>>>> /g13/0=t;1=t\n
>>>>>>> /g14/0=t;1=t\n
>>>>>>> /g15/0=t;1=t\n
>>>>>>> /g16/0=t;1=t\n
>>>>>>> /g17/0=t;1=t\n
>>>>>>> /g18/0=t;1=t\n
>>>>>>> /g19/0=t;1=t\n
>>>>>>> /g20/0=t;1=t\n
>>>>>>> /g21/0=t;1=t\n
>>>>>>> /g22/0=t;1=t\n
>>>>>>> /g23/0=t;1=t\n
>>>>>>> /g24/0=t;1=t\n
>>>>>>> /g25/0=t;1=t\n
>>>>>>> /g26/0=t;1=t\n
>>>>>>> /g27/0=t;1=t\n
>>>>>>> /g28/0=t;1=t\n
>>>>>>> /g29/0=t;1=t\n
>>>>>>> /g30/0=t;1=t\n
>>>>>>> /g31/0=t;1=t\n'
>>>>>>>
>>>>>>> My ultimate goal is for a thread bound to a particular domain to be
>>>>>>> able to unassign and reassign the local domain's 32 counters in a
>>>>>>> single write() with no IPIs at all. And when IPIs are required, then
>>>>>>> no more than one per domain, regardless of the number of groups
>>>>>>> updated.
>>>>>>>
>>>>>>
>>>>>> Yes. I think I got the idea. Thanks.
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> struct resctrl_monitor_cfg {
>>>>>>>>>       int closid;
>>>>>>>>>       int rmid;
>>>>>>>>>       int evtid;
>>>>>>>>>       bool dirty;
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> This mirrors the info needed in whatever register configures the
>>>>>>>>> counter, plus a dirty flag to skip over the ones that don't need to be
>>>>>>>>> updated.
>>>>>>>>
>>>>>>>> This is what my understanding of your implementation.
>>>>>>>>
>>>>>>>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>>>>>>>> index d94abba1c716..9cebf065cc97 100644
>>>>>>>> --- a/include/linux/resctrl.h
>>>>>>>> +++ b/include/linux/resctrl.h
>>>>>>>> @@ -94,6 +94,13 @@ struct rdt_ctrl_domain {
>>>>>>>>           u32                             *mbps_val;
>>>>>>>>    };
>>>>>>>>
>>>>>>>> +struct resctrl_monitor_cfg {
>>>>>>>> +    int closid;
>>>>>>>> +    int rmid;
>>>>>>>> +    int evtid;
>>>>>>>> +    bool dirty;
>>>>>>>> +};
>>>>>>>> +
>>>>>>>>    /**
>>>>>>>>     * struct rdt_mon_domain - group of CPUs sharing a resctrl monitor
>>>>>>>> resource
>>>>>>>>     * @hdr:               common header for different domain types
>>>>>>>> @@ -116,6 +123,7 @@ struct rdt_mon_domain {
>>>>>>>>           struct delayed_work             cqm_limbo;
>>>>>>>>           int                             mbm_work_cpu;
>>>>>>>>           int                             cqm_work_cpu;
>>>>>>>> +     /* Allocate num_mbm_cntrs entries in each domain */
>>>>>>>> +       struct resctrl_monitor_cfg      *mon_cfg;
>>>>>>>>    };
>>>>>>>>
>>>>>>>>
>>>>>>>> When a user requests an assignment for total event to the default group
>>>>>>>> for domain 0, you go search in rdt_mon_domain(dom 0) for empty mon_cfg
>>>>>>>> entry.
>>>>>>>>
>>>>>>>> If there is an empty entry, then use that entry for assignment and
>>>>>>>> update closid, rmid, evtid and dirty = 1. We can get all these
>>>>>>>> information from default group here.
>>>>>>>>
>>>>>>>> Does this make sense?
>>>>>>>
>>>>>>> Yes, sounds correct.
>>>>>>
>>>>>> I will probably add cntr_id in resctrl_monitor_cfg structure and
>>>>>> initialize during the allocation. And rename the field 'dirty' to
>>>>>> 'active'(or something similar) to hold the assign state for that
>>>>>> entry. That way we have all the information required for assignment
>>>>>> at one place. We don't need to update the rdtgroup structure.
>>>>>>
>>>>>> Reinette, What do you think about this approach?
>>>>>
>>>>> I think this approach is in the right direction. Thanks to Peter for
>>>>> the guidance here.
>>>>> I do not think that it is necessary to add cntr_id to resctrl_monitor_cfg
>>>>> though, I think the cntr_id would be the index to the array instead?
>>>>
>>>> Yes. I think We can use the index as cntn_id. Will let you know otherwise.
>>>>
>>>>
>>>>>
>>>>> It may also be worthwhile to consider using a pointer to the resource
>>>>> group instead of storing closid and rmid directly. If used to indicate
>>>>> initialization then an initialized pointer is easier to distinguish than
>>>>> the closid/rmid that may have zero as valid values.
>>>>
>>>> Sure. Sounds good.
>>>>
>>>>>
>>>>> I expect evtid will be enum resctrl_event_id and that raises the question
>>>>> of whether "0" can indeed be used as an "uninitialized" value since doing
>>>>> so would change the meaning of the enum. It may indeed keep things
>>>>> separated by maintaining evtid as an enum resctrl_event_id and note the
>>>>> initialization differently ... either via a pointer to a resource group
>>>>> or entirely separately as Babu indicates later.
>>>>
>>>> Sure. Will add evtid as enum resctrl_event_id and use the "state" to
>>>> indicate assign/unassign/dirty status.
>>>
>>> Is "assign/unassign" state needed? If resctrl_monitor_cfg contains a pointer
>>> to the resource group to which the counter has been assigned then I expect NULL
>>> means unassigned and a value means assigned?
>>
>> Yes. We use the rdtgroup pointer to check the assign/unassign state.
>>
>> I will drop the 'state' field. Peter can add state when he wants use it
>> for optimization later.
>>
>> I think we need to have the 'cntr_id" field here in resctrl_monitor_cfg.
>> When we access the pointer from mbm_state, we wont know what is cntr_id
>> index it came from.
>>
> 
> oh, good point. I wonder how Peter addressed this in his PoC. As an alternative,
> could the cntr_id be used in mbm_state instead of a pointer? 
> 

Yes. It can be done.

I thought it would be better to have everything at once place.

struct resctrl_monitor_cfg {
  unsigned int            cntr_id;
  enum resctrl_event_id   evtid;
  struct rdtgroup         *rgtgrp;
};

This will have everything required to assign/unassign the event.

Thanks
Babu Moger

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