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Thu, 1 Aug 2024 21:49:47 +0000 Message-ID: <05b4e345-ad14-4ea9-a13f-2c9b3a6eb422@intel.com> Date: Thu, 1 Aug 2024 14:49:43 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 00/20] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC) To: , , , , , , CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <1c50b589-a738-4ae6-8362-bd1ce0d0dc98@amd.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <1c50b589-a738-4ae6-8362-bd1ce0d0dc98@amd.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR04CA0115.namprd04.prod.outlook.com (2603:10b6:303:83::30) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|MW4PR11MB5870:EE_ X-MS-Office365-Filtering-Correlation-Id: 0b177f92-ccc0-4b9d-fcf8-08dcb273dca0 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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Keeping the current interface, users can enable a maximum >>> of 2 ABMC counters per group. User will also have the option to enable only >>> one counter to the group. If the system runs out of assignable ABMC >>> counters, kernel will display an error. Users need to disable an already >>> enabled counter to make space for new assignments. >> >> The implementation appears to be converging on an interface that can >> be generic enough to be used by other features discussed along the way. >> "Linux implementation" summary can thus add: >> >>     Create a generic interface aimed to support user space assignment >>     of scarce counters used for monitoring. First usage of interface >>     is by ABMC with option to expand usage to "soft-RMID" and MPAM >>     counters in future. > > Sure. > >> >> >>> # Examples >>> >>> a. Check if ABMC support is available >>>     #mount -t resctrl resctrl /sys/fs/resctrl/ >>> >>>     #cat /sys/fs/resctrl/info/L3_MON/mbm_mode >>>     [abmc] >>>     legacy >>> >>>     Linux kernel detected ABMC feature and it is enabled. >> >> How about renaming "abmc" to "mbm_cntrs"? This will match the num_mbm_cntrs >> info file and be the final step to make this generic so that another >> architecture >> can more easily support assignining hardware counters without needing to call >> the feature AMD's "abmc". > > I think we aleady settled this with "mbm_cntr_assignable". > > For soft-RMID" it will be mbm_sw_assignable. Maybe getting a bit long but how about "mbm_cntr_sw_assignable" to match with the term "mbm_cntr" in accompanying "num_mbm_cntrs"? >> Expanding on this it may be possible to add a new "sw_mbm_cntrs" feature that >> will be the "soft-RMID" feature while also reflecting the "mbm_cntrs" name >> so that when user space enables that feature its properties can be found in >> "num_mbm_cntrs". >> >> The "abmc" kernel parameter remains but that does seem separate from this >> resctrl fs feature since it is explicitly tied to X86_FEATURE_ABMC surely >> making it architecture specific. >> >>> >>> b. Check how many ABMC counters are available. >>> >>>     #cat /sys/fs/resctrl/info/L3_MON/num_cntrs >>>     32 >> >> This is now num_mbm_cntrs > > Sure. > >> >>> >>> c. Create few resctrl groups. >>> >>>     # mkdir /sys/fs/resctrl/mon_groups/child_default_mon_grp >>>     # mkdir /sys/fs/resctrl/non_default_ctrl_mon_grp >>>     # mkdir >>> /sys/fs/resctrl/non_default_ctrl_mon_grp/mon_groups/child_non_default_mon_grp >>> >>> >>> d. This series adds a new interface file >>> /sys/fs/resctrl/info/L3_MON/mbm_control >>>     to list and modify the group's monitoring states. File provides >>> single place >>>     to list monitoring states of all the resctrl groups. It makes it >>> easier for >>>     user space to learn about the counters are used without needing to >>> traverse >>>     all the groups thus reducing the number of filesystem calls. >>> >>>     The list follows the following format: >>> >>>     "//=" >>> >>>     Format for specific type of groups: >>> >>>     * Default CTRL_MON group: >>>      "//=" >>> >>>         * Non-default CTRL_MON group: >>>                 "//=" >>> >>>         * Child MON group of default CTRL_MON group: >>>                 "//=" >>> >>>         * Child MON group of non-default CTRL_MON group: >>>                 "//=" >>> >>>         Flags can be one of the following: >>> >>>          t  MBM total event is enabled. >>>          l  MBM local event is enabled. >>>          tl Both total and local MBM events are enabled. >>>          _  None of the MBM events are enabled >> >> The language needs to be changed here (and in the many copied places) to >> be specific about what setting the flag accomplishes. For example, in >> "legacy" mode user space can be expected to find all events enabled, no? >> Needing a new feature to set a flag to accomplish something that is >> possible in legacy mode can thus cause confusion. > > Yes. It is possible to do it. But I feel unnessassary. > >> >> If I understand the implementation reading "mbm_control" will fail >> if system is ABMC capable but it is disabled. Why can "mbm_control" not >> always be displayed to user space? For example, what if "mbm_control" is >> always available to user space and it can provide specific information to >> user space. For example: >>     t  MBM total event is enabled but may not always be counted. >>     T  MBM total event is enabled and being counted. >> >> On AMD systems resource groups will have "t" associated with monitor >> groups when ABMC disabled, "T" when ABMC enabled and a counter assigned. >> On Intel systems monitor groups will always have "T". > > I think more flags will add more confusion. > >> >> For "soft-RMID" the flag could possible continue to be "T"? >> >> I am trying to find ways to communicate to user space consistently >> and clearly and any insights will be appreciated. We really do not want >> to add this interface and then find that it just causes confusion. >> >> It is not quite obvious to me when the new files should be visible and >> what they should present to the user. "mbm_mode" is now always visible. >> Should "num_mbm_cntrs" not also always be visible? Right now "num_mbm_cntrs" >> appears to be only associated to ABMC, should it not also, for example, >> be the file that "soft-RMID" may use to share how many counters are >> available? Its contents will thus be dynamic based on which "MBM mode" is >> active, begging the question, what should it contain when "legacy" mode is >> enabled, should "num_mbm_cntrs" perhaps show "0" to user space when >> "legacy" mode is active? > > Its good we have this discussion. > > How about we go with simple way for now. The mbm_mode will only available > when ABMC or Soft_RMID(MPAM feature) is supported. Same way for the > num_mbm_cntrs. If ABMC or Soft_RMID is supported then user can still enable "legacy" instead. What will num_mbm_cntrs and mbm_control display when user enables "legacy"? Reinette