linux-kernel - Re: [PATCH v3 06/10] x86/resctrl: Introduce mon_configurable to detect Bandwidth Monitoring Event Configuration

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Date:   Thu, 25 Aug 2022 15:44:20 -0500
From:   "Moger, Babu" <bmoger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, babu.moger@....com,
        fenghua.yu@...el.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de
Cc:     eranian@...gle.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, corbet@....net, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, bagasdotme@...il.com,
        "Luck, Tony" <tony.luck@...el.com>
Subject: Re: [PATCH v3 06/10] x86/resctrl: Introduce mon_configurable to
 detect Bandwidth Monitoring Event Configuration


On 8/25/2022 10:56 AM, Reinette Chatre wrote:
> Hi Babu,
>
> On 8/25/2022 8:11 AM, Moger, Babu wrote:
>> On 8/24/22 16:15, Reinette Chatre wrote:
>>> On 8/22/2022 6:43 AM, Babu Moger wrote:
>>>> Newer AMD processors support the new feature Bandwidth Monitoring Event
>>>> Configuration (BMEC). The events mbm_total_bytes and mbm_local_bytes
>>>> are configurable when this feature is present.
>>>>
>>>> Set mon_configurable if the feature is available.
>>>>
> ...
>
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> index fc5286067201..855483b297a8 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>> @@ -995,6 +995,16 @@ static int rdt_num_rmids_show(struct kernfs_open_file *of,
>>>>   	return 0;
>>>>   }
>>>>   
>>>> +static int rdt_mon_configurable_show(struct kernfs_open_file *of,
>>>> +				     struct seq_file *seq, void *v)
>>>> +{
>>>> +	struct rdt_resource *r = of->kn->parent->priv;
>>>> +
>>>> +	seq_printf(seq, "%d\n", r->mon_configurable);
>>> Why is this file needed? It seems that the next patches also introduce
>>> files in support of this new feature that will make the actual configuration
>>> data accessible - those files are only created if this feature is supported.
>>> Would those files not be sufficient for user space to learn about the feature
>>> support?
>> This is part of /sys/fs/resctrl/info/L3_MON# directory which basically has
>> the information about all the monitoring features. As this is one of the
>> mon features, I have added it there. Also, this can be used from the
>> utility(like pqos or rdtset) to check if the configuring the monitor is
>> supported without looking at individual files. It makes things easier.
> I understand the motivation. My concern is that this is a resource wide
> file that will display a binary value that, if true, currently means two
> events are configurable. We need to consider how things can change in the
> future. We should consider that this is only the beginning of monitoring
> configuration and need this interface to be ready for future changes. For
> example, what if all of the monitoring events are configurable? Let's say,
> for example, in future AMD hardware the "llc_occupancy" event also becomes
> configurable, how should info/L3_MON/configurable be interpreted? On some
> machines it would thus mean that mbm_total_bytes and mbm_local_bytes are
> configurable and on some machines it would mean that mbm_total_bytes,
> mbm_local_bytes, and llc_occupancy are configurable. This does not make
> it easy for utilities.
>
> So, in this series the info directory on a system that supports BMEC
> would look like:
>
> info/L3_MON/mon_features:llc_occupancy
> info/L3_MON/mon_features:mbm_total_bytes
> info/L3_MON/mon_features:mbm_local_bytes
> info/L3_MON/configurable:1
>
> Would information like below not be more specific?
> info/L3_MON/mon_features:llc_occupancy
> info/L3_MON/mon_features:mbm_total_bytes
> info/L3_MON/mon_features:mbm_local_bytes
> info/L3_MON/mon_features:mbm_total_config
> info/L3_MON/mon_features:mbm_local_config

Hi Reinette,

  Yes. That is more specific.

So, basically your idea is to print the information from mon_evt 
structure if mon_configarable is set in the resource structure.

Some thing like ..

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c 
b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 83c8780726ff..46c6bf3f08e3 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -1194,8 +1194,11 @@ static int rdt_mon_features_show(struct 
kernfs_open_file *of,
         struct rdt_resource *r = of->kn->parent->priv;
         struct mon_evt *mevt;

-       list_for_each_entry(mevt, &r->evt_list, list)
+       list_for_each_entry(mevt, &r->evt_list, list) {
                 seq_printf(seq, "%s\n", mevt->name);
+               if (r->mon_configurable)
+                       seq_printf(seq, "%s\n", mevt->config);
+       }

         return 0;
  }

Is that the idea?

Thanks
Babu