Re: [PATCH v11 13/13] Documentation/x86: Update resctrl.rst for new features

["Moger, Babu" <bmoger@amd.com>]

Hi Reinette,

On 1/11/2023 4:56 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 1/11/2023 2:39 PM, Moger, Babu wrote:
>> [AMD Official Use Only - General]
>>
>> Hi Reinette,
>>
>>> -----Original Message-----
>>> From: Reinette Chatre <reinette.chatre@intel.com>
>>> Sent: Wednesday, January 11, 2023 4:07 PM
>>> To: Moger, Babu <Babu.Moger@amd.com>; corbet@lwn.net;
>>> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de
>>> Cc: fenghua.yu@intel.com; dave.hansen@linux.intel.com; x86@kernel.org;
>>> hpa@zytor.com; paulmck@kernel.org; akpm@linux-foundation.org;
>>> quic_neeraju@quicinc.com; rdunlap@infradead.org;
>>> damien.lemoal@opensource.wdc.com; songmuchun@bytedance.com;
>>> peterz@infradead.org; jpoimboe@kernel.org; pbonzini@redhat.com;
>>> chang.seok.bae@intel.com; pawan.kumar.gupta@linux.intel.com;
>>> jmattson@google.com; daniel.sneddon@linux.intel.com; Das1, Sandipan
>>> <Sandipan.Das@amd.com>; tony.luck@intel.com; james.morse@arm.com;
>>> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org;
>>> bagasdotme@gmail.com; eranian@google.com; christophe.leroy@csgroup.eu;
>>> jarkko@kernel.org; adrian.hunter@intel.com; quic_jiles@quicinc.com;
>>> peternewman@google.com
>>> Subject: Re: [PATCH v11 13/13] Documentation/x86: Update resctrl.rst for new
>>> features
>>>
>>> Hi Babu,
>>>
>>> On 1/9/2023 8:44 AM, Babu Moger wrote:
>>>> Update the documentation for the new features:
>>>> 1. Slow Memory Bandwidth allocation (SMBA).
>>>>     With this feature, the QOS  enforcement policies can be applied
>>>>     to the external slow memory connected to the host. QOS enforcement
>>>>     is accomplished by assigning a Class Of Service (COS) to a processor
>>>>     and specifying allocations or limits for that COS for each resource
>>>>     to be allocated.
>>>>
>>>> 2. Bandwidth Monitoring Event Configuration (BMEC).
>>>>     The bandwidth monitoring events mbm_total_bytes and mbm_local_bytes
>>>>     are set to count all the total and local reads/writes respectively.
>>>>     With the introduction of slow memory, the two counters are not
>>>>     enough to count all the different types of memory events. With the
>>>>     feature BMEC, the users have the option to configure mbm_total_bytes
>>>>     and mbm_local_bytes to count the specific type of events.
>>>>
>>>> Also add configuration instructions with examples.
>>>>
>>>> Signed-off-by: Babu Moger <babu.moger@amd.com>
>>>> ---
>>>>   Documentation/x86/resctrl.rst | 142
>>>> +++++++++++++++++++++++++++++++++-
>>>>   1 file changed, 140 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/Documentation/x86/resctrl.rst
>>>> b/Documentation/x86/resctrl.rst index 71a531061e4e..2860856f4463
>>>> 100644
>>>> --- a/Documentation/x86/resctrl.rst
>>>> +++ b/Documentation/x86/resctrl.rst
>>>> @@ -17,14 +17,16 @@ AMD refers to this feature as AMD Platform Quality
>>> of Service(AMD QoS).
>>>>   This feature is enabled by the CONFIG_X86_CPU_RESCTRL and the x86
>>>> /proc/cpuinfo  flag bits:
>>>>
>>>> -=============================================
>>> 	================================
>>>> +===============================================
>>> 	================================
>>>>   RDT (Resource Director Technology) Allocation	"rdt_a"
>>>>   CAT (Cache Allocation Technology)		"cat_l3", "cat_l2"
>>>>   CDP (Code and Data Prioritization)		"cdp_l3", "cdp_l2"
>>>>   CQM (Cache QoS Monitoring)			"cqm_llc",
>>> "cqm_occup_llc"
>>>>   MBM (Memory Bandwidth Monitoring)		"cqm_mbm_total",
>>> "cqm_mbm_local"
>>>>   MBA (Memory Bandwidth Allocation)		"mba"
>>>> -=============================================
>>> 	================================
>>>> +SMBA (Slow Memory Bandwidth Allocation)         "smba"
>>>> +BMEC (Bandwidth Monitoring Event Configuration) "bmec"
>>>> +===============================================
>>> 	================================
>>> I expect that you will follow Boris's guidance here and not make these flags
>>> visible in /proc/cpuinfo. That would imply that this addition will have no entries
>>> in the second column. Perhaps this could be made easier to parse by using
>>> empty quotes ("") in the second column to match syntax used in the existing
>>> flags as well as the cpufeatures.h change?
>> Hmm.. I thought we dropped that idea for now. Did I miss understand that?
> I referred to the guidance in https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2FY7xjxUj%2BKnOEJssZ%40zn.tnic%2F&data=05%7C01%7CBabu.Moger%40amd.com%7C900eb41c0e6049dd342208daf4270d2b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638090745842366944%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=%2F5GVOhnxq1%2B3nJwGtlApvLfC%2FeX3X9RDaUZa9R92NiY%3D&reserved=0
> Since the SMBA and BMEC features have never appeared in /proc/cpuinfo there cannot
> be a user space that expects these flags in /proc/cpuinfo and thus no risk of
> breaking user space. User space can get information about SMBA and BMEC
> from the info directory.
ok. Got it.
>
> Later that thread discussed removal of existing resctrl feature flags from
> /proc/cpuinfo - that is what I think we shouldn't do since there are
> user space consumers of those flags. I thus agree that the task described in
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2FMW3PR12MB455384130AF0BDE3AF88BCF095FE9%40MW3PR12MB4553.namprd12.prod.outlook.com%2F&data=05%7C01%7CBabu.Moger%40amd.com%7C900eb41c0e6049dd342208daf4270d2b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638090745842366944%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=kE7d0cFYyJq1n4ZKKeeF%2FC%2FFDDJy0Sc%2Fd5MZ%2Bc56WQw%3D&reserved=0
> can be dropped.
Sure.
> I do not think this is a big change ... just add the empty quotes to the
> two cpufeatures.h patches and a new snippet to the resctrl documentation.


How about this? I want to get this right.. Hopefully next version can be 
final.

diff --git a/Documentation/x86/resctrl.rst b/Documentation/x86/resctrl.rst
index 2860856f4463..7df5889237f4 100644
--- a/Documentation/x86/resctrl.rst
+++ b/Documentation/x86/resctrl.rst
@@ -24,10 +24,15 @@ CDP (Code and Data Prioritization) "cdp_l3", "cdp_l2"
  CQM (Cache QoS Monitoring)                     "cqm_llc", "cqm_occup_llc"
  MBM (Memory Bandwidth Monitoring)              "cqm_mbm_total", 
"cqm_mbm_local"
  MBA (Memory Bandwidth Allocation)              "mba"
-SMBA (Slow Memory Bandwidth Allocation)         "smba"
-BMEC (Bandwidth Monitoring Event Configuration) "bmec"
+SMBA (Slow Memory Bandwidth Allocation)         ""
+BMEC (Bandwidth Monitoring Event Configuration) ""
  =============================================== 
================================

+Historically, new features were made visible by default in 
/proc/cpuinfo. This
+resulted in the feature flags becoming hard to parse by the humans. 
Adding a new
+flag to /proc/cpuinfo should be avoided if user space can obtain 
information
+about the feature from resctrl's info directory.
+
  To use the feature mount the file system::

   # mount -t resctrl resctrl [-o cdp[,cdpl2][,mba_MBps]] /sys/fs/resctrl


thanks

Babu