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I wonder if we could perhaps always expose num_mbm_cntrs as a way for >>> users to know if their platform may be impacted by this type of "unpredictability" (by comparing >>> num_mbm_cntrs to num_rmids). >> >> There is some round about(or hacky) way to find that out number of RMIDs >> that can be active. > > Does this give consistent and accurate data? Is this something that can be added to resctrl? > (Reading your other message [1] it does not sound as though it can produce an accurate > number on boot.) > If not then it will be up to the documentation to be accurate. > > >>>> + >>>> + AMD Platforms with ABMC (Assignable Bandwidth Monitoring Counters) feature >>>> + enable this mode by default so that counters remain assigned even when the >>>> + corresponding RMID is not in use by any processor. >>>> + >>>> + "default": >>>> + >>>> + In default mode resctrl assumes there is a hardware counter for each >>>> + event within every CTRL_MON and MON group. Reading mbm_total_bytes or >>>> + mbm_local_bytes may report 'Unavailable' if there is no counter associated >>>> + with that event. >>> >>> If I understand correctly, on AMD platforms without ABMC the events only report >>> "Unavailable" if there is no counter assigned at the time of the query. If a counter >>> is unassigned and then reassigned then the event count will reset and the user >>> will get some data back but it may thus be unpredictable (to match earlier language). >>> Is this correct? Any AMD platform in "default" mode may thus be vulnerable to >>> "unpredictable" event counts (not just "Unavailable") ... this gets complicated >> >> Yes. All the AMD systems without ABMC are affected by this problem. >> >>> because users should be steered to avoid "default" mode if mbm_assign_mode is >>> available, while not be made concerned to use "default" mode on Intel where >>> mbm_assign_mode is not available. >> >> Can we add text to clarify this? > > Please do. I think we need to add text about AMD systems. How about this? "default": In default mode resctrl assumes there is a hardware counter for each event within every CTRL_MON and MON group. On AMD systems with 16 more monitoring groups, reading mbm_total_bytes or mbm_local_bytes may report 'Unavailable' if there is no counter associated with that event. It is therefore recommended to use the 'mbm_cntr_assign' mode, if supported." > > Reinette > > [1] https://lore.kernel.org/all/35fc70fd-0281-4ac8-b32b-efa2f4516901@amd.com/ > -- - Babu Moger