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Date: Thu, 1 Feb 2024 21:01:18 -0800 From: Reinette Chatre <reinette.chatre@...el.com> To: Babu Moger <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com> CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>, <rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>, <yanjiewtw@...il.com>, <kim.phillips@....com>, <lukas.bulwahn@...il.com>, <seanjc@...gle.com>, <jmattson@...gle.com>, <leitao@...ian.org>, <jpoimboe@...nel.org>, <rick.p.edgecombe@...el.com>, <kirill.shutemov@...ux.intel.com>, <jithu.joseph@...el.com>, <kai.huang@...el.com>, <kan.liang@...ux.intel.com>, <daniel.sneddon@...ux.intel.com>, <pbonzini@...hat.com>, <sandipan.das@....com>, <ilpo.jarvinen@...ux.intel.com>, <peternewman@...gle.com>, <maciej.wieczor-retman@...el.com>, <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <eranian@...gle.com>, James Morse <james.morse@....com> Subject: Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC) +James On 2/1/2024 8:09 PM, Reinette Chatre wrote: > Hi Babu, > > On 1/19/2024 10:22 AM, Babu Moger wrote: >> >> These series adds the support for Assignable Bandwidth Monitoring Counters > > Not a good start ([1]). > >> (ABMC). It is also called QoS RMID Pinning feature >> >> The feature details are documented in the APM listed below [1]. >> [1] AMD64 Architecture Programmer's Manual Volume 2: System Programming >> Publication # 24593 Revision 3.41 section 19.3.3.3 Assignable Bandwidth >> Monitoring (ABMC). The documentation is available at >> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 >> >> The patches are based on top of commit >> 1ac6b49423e83af2abed9be7fbdf2e491686c66b (tip/master) >> >> # Introduction >> >> AMD hardware can support 256 or more RMIDs. However, bandwidth monitoring >> feature only guarantees that RMIDs currently assigned to a processor will >> be tracked by hardware. The counters of any other RMIDs which are no longer >> being tracked will be reset to zero. The MBM event counters return >> "Unavailable" for the RMIDs that are not active. >> >> Users can create 256 or more monitor groups. But there can be only limited >> number of groups that can be give guaranteed monitoring numbers. With ever > > "can be given"? > >> changing configurations there is no way to definitely know which of these >> groups will be active for certain point of time. Users do not have the >> option to monitor a group or set of groups for certain period of time >> without worrying about RMID being reset in between. >> >> The ABMC feature provides an option to the user to assign an RMID to the >> hardware counter and monitor the bandwidth for a longer duration. >> The assigned RMID will be active until the user unassigns it manually. >> There is no need to worry about counters being reset during this period. >> Additionally, the user can specify a bitmask identifying the specific >> bandwidth types from the given source to track with the counter. >> >> Without ABMC enabled, monitoring will work in current mode without >> assignment option. >> >> # Linux Implementation >> >> Linux resctrl subsystem provides the interface to count maximum of two >> memory bandwidth events per group, from a combination of available total >> and local events. Keeping the current interface, users can assign a maximum >> of 2 ABMC counters per group. User will also have the option to assign only >> one counter to the group. If the system runs out of assignable ABMC >> counters, kernel will display an error. Users need to unassign an already >> assigned counter to make space for new assignments. >> >> >> # Examples >> >> a. Check if ABMC support is available >> #mount -t resctrl resctrl /sys/fs/resctrl/ >> >> #cat /sys/fs/resctrl/info/L3_MON/mon_features >> llc_occupancy >> mbm_total_bytes >> mbm_total_bytes_config >> mbm_local_bytes >> mbm_local_bytes_config >> mbm_assign_capable ← Linux kernel detected ABMC feature >> >> b. Check if ABMC is enabled. By default, ABMC feature is disabled. >> Monitoring works in legacy monitor mode when ABMC is not enabled. >> >> #cat /sys/fs/resctrl/info/L3_MON/mbm_assign_enable >> 0 >> > > With the introduction of "mbm_assign_enable" the entry in mon_features seems > to provide duplicate information. > >> c. There will be new file "monitor_state" for each monitor group when ABMC >> feature is supported. However, monitor_state is not available if ABMC is >> disabled. >> >> #cat /sys/fs/resctrl/monitor_state >> Unsupported > > This sounds potentially confusing since users will still be able to monitor > the groups ... > >> >> d. Read the event mbm_total_bytes and mbm_local_bytes. Without ABMC >> enabled, monitoring will work in current mode without assignment option. >> >> # cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_total_bytes >> 779247936 >> # cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_local_bytes >> 765207488 >> >> e. Enable ABMC mode. >> >> #echo 1 > /sys/fs/resctrl/info/L3_MON/mbm_assign_enable >> #cat /sys/fs/resctrl/info/L3_MON/mbm_assign_enable >> 1 >> >> f. Read the monitor states. By default, both total and local MBM >> events are in "unassign" state. >> >> #cat /sys/fs/resctrl/monitor_state >> total=unassign;local=unassign > > This interface does not seem to take into account that hardware > can support assignment per domain. I understand that this is > not something you want to implement at this time but the user interface > has to accommodate such an enhancement. This was already mentioned, and > you did acknowledge the point [3] to this new version that does not > reflect this is unexpected. > > My previous suggestions do seem to still stand and and I also am not able to > see how Peter's requests [2] were considered. This same interface needs to > accommodate usages apart from ABMC. For example, how to use this interface > to address the same counter issue on AMD hardware without ABMC, and MPAM > (pending James's feedback). > > I understand that until we hear from Arm we do not know all the requirements > that this interface needs to support, but I do expect this interface to > at least consider requirements and usage scenarios that are already known. > >> g. Read the event mbm_total_bytes and mbm_local_bytes. In ABMC mode, >> the MBA events are not available until the user assigns the events >> explicitly. >> >> #cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_total_bytes >> Unsupported >> >> #cat /sys/fs/resctrl/mon_data/mon_L3_00/mbm_local_bytes >> Unsupported >> > > This needs some more thought to accommodate Peter's scenario where the counter > can be expected to return the final count after the counter is disabled. > >> h. The event llc_occupancy is not affected by ABMC mode. Users can still >> read the llc_occupancy. >> >> #cat /sys/fs/resctrl/mon_data/mon_L3_00/llc_occupancy >> 557056 >> >> i. Now assign the total event and read the monitor_state. >> >> #echo total=assign > /sys/fs/resctrl/monitor_state >> #cat /sys/fs/resctrl/monitor_state >> total=assign;local=unassign >> > > I do not see the "global assign/unassign" scenario addressed. > > This version seems to ignore (without discussion) a lot of earlier > feedback. > > Reinette > > [1] https://lore.kernel.org/lkml/5ce67d8f-e207-4029-8fb3-0bc7deab1e9f@amd.com/ > [2] https://lore.kernel.org/lkml/CALPaoCiRD6j_Rp7ffew+PtGTF4rWDORwbuRQqH2i-cY5SvWQBg@mail.gmail.com/ > [3] https://lore.kernel.org/lkml/38421428-84cb-b67e-f3ce-b7a0233e016b@amd.com/
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