linux-kernel - Re: [PATCH v11 00/23] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <bdb48b29-92cf-4f98-b375-25ec84f0941a@amd.com>
Date: Mon, 17 Feb 2025 10:45:29 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Peter Newman <peternewman@...gle.com>,
 Reinette Chatre <reinette.chatre@...el.com>
Cc: "Moger, Babu" <bmoger@....com>, Dave Martin <Dave.Martin@....com>,
 corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, tony.luck@...el.com, x86@...nel.org,
 hpa@...or.com, paulmck@...nel.org, akpm@...ux-foundation.org,
 thuth@...hat.com, rostedt@...dmis.org, xiongwei.song@...driver.com,
 pawan.kumar.gupta@...ux.intel.com, daniel.sneddon@...ux.intel.com,
 jpoimboe@...nel.org, perry.yuan@....com, sandipan.das@....com,
 kai.huang@...el.com, xiaoyao.li@...el.com, seanjc@...gle.com,
 xin3.li@...el.com, andrew.cooper3@...rix.com, ebiggers@...gle.com,
 mario.limonciello@....com, james.morse@....com, tan.shaopeng@...itsu.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 maciej.wieczor-retman@...el.com, eranian@...gle.com
Subject: Re: [PATCH v11 00/23] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)

Hi All,

On 2/17/25 04:26, Peter Newman wrote:
> Hi Reinette,
> 
> On Fri, Feb 14, 2025 at 8:18 PM Reinette Chatre
> <reinette.chatre@...el.com> wrote:
>>
>> Hi Babu,
>>
>> On 2/14/25 10:31 AM, Moger, Babu wrote:
>>> On 2/14/2025 12:26 AM, Reinette Chatre wrote:
>>>> On 2/13/25 9:37 AM, Dave Martin wrote:
>>>>> On Wed, Feb 12, 2025 at 03:33:31PM -0800, Reinette Chatre wrote:
>>>>>> On 2/12/25 9:46 AM, Dave Martin wrote:
>>>>>>> On Wed, Jan 22, 2025 at 02:20:08PM -0600, Babu Moger wrote:
>>
>> (quoting relevant parts with goal to focus discussion on new possible syntax)
>>
>>>>>> I see the support for MPAM events distinct from the support of assignable counters.
>>>>>> Once the MPAM events are sorted, I think that they can be assigned with existing interface.
>>>>>> Please help me understand if you see it differently.
>>>>>>
>>>>>> Doing so would need to come up with alphabetical letters for these events,
>>>>>> which seems to be needed for your proposal also? If we use possible flags of:
>>>>>>
>>>>>> mbm_local_read_bytes a
>>>>>> mbm_local_write_bytes b
>>>>>>
>>>>>> Then mbm_assign_control can be used as:
>>>>>> # echo '//0=ab;1=b' >/sys/fs/resctrl/info/L3_MON/mbm_assign_control
>>>>>> # cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_local_read_bytes
>>>>>> <value>
>>>>>> # cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_local_bytes
>>>>>> <sum of mbm_local_read_bytes and mbm_local_write_bytes>
>>>>>>
>>>>>> One issue would be when resctrl needs to support more than 26 events (no more flags available),
>>>>>> assuming that upper case would be used for "shared" counters (unless this interface is defined
>>>>>> differently and only few uppercase letters used for it). Would this be too low of a limit?
>>
>> As mentioned above, one possible issue with existing interface is that
>> it is limited to 26 events (assuming only lower case letters are used). The limit
>> is low enough to be of concern.
> 
> The events which can be monitored by a single counter on ABMC and MPAM
> so far are combinable, so 26 counters per group today means it limits
> breaking down MBM traffic for each group 26 ways. If a user complained
> that a 26-way breakdown of a group's MBM traffic was limiting their
> investigation, I would question whether they know what they're looking
> for.

Based on the discussion so far, it felt like it is not a group level
breakdown. It is kind of global level breakdown. I could be wrong here.

My understanding so far, MPAM has a number of global counters. It can be
assigned to any domain in the system and monitor events.

They also have a way to configure the events (read, write or both).

Both these feature are inline with current resctrl implementation and can
be easily adapted.

One thing I am not clear why MPAM implementation plans to create separate
files(dynamically) in /sys/fs/resctrl/info/L3_MON/ directory to read the
events. We already have files in each group to read the events.

# ls -l /sys/fs/resctrl/mon_data/mon_L3_00/
total 0
-r--r--r--. 1 root root 0 Feb 17 08:16 llc_occupancy
-r--r--r--. 1 root root 0 Feb 17 08:16 mbm_local_bytes
-r--r--r--. 1 root root 0 Feb 17 08:16 mbm_total_bytes

-- 
Thanks
Babu Moger