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Sat, 1 Aug 2026 23:47:03 +0000 Message-ID: <5f10ceab-5569-409b-8e78-c4a4f232489f@intel.com> Date: Sat, 1 Aug 2026 16:47:01 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot To: Borislav Petkov CC: "Luck, Tony" , Babu Moger , "Moger, Babu" , "x86@kernel.org" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "corbet@lwn.net" , "skhan@linuxfoundation.org" , "tglx@kernel.org" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "linux-kernel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "Eranian, Stephane" , "peternewman@google.com" References: <7495c998-4f3f-4ea5-a383-350584afb42c@amd.com> <6d006c4c-907a-435f-ac94-32974562ce2d@intel.com> <20260730184626.GIamucAtZxMUUGagGm@fat_crate.local> <7d3dd384-9101-49cb-ada4-b8320198233e@amd.com> <20260730194328.GEamupYLsURg6VvxBU@fat_crate.local> <20260731003436.GKamvtnJlKydaEqstA@fat_crate.local> <4bfafa1e-a7da-4c37-8da9-9fbba14106fd@intel.com> <20260801012227.GEam1KUyMz601QOzrT@fat_crate.local> From: Reinette Chatre Content-Language: en-US In-Reply-To: <20260801012227.GEam1KUyMz601QOzrT@fat_crate.local> Content-Type: text/plain; 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AMD PQoS (whether it supports ABMC or not) also exposes an RMID limit that is used as the upper limit for the number of monitor groups that can be tracked by the hardware. > I mean, it is kinda natural behavior to track counter occupancy and deny the > allocation of new ones if there are none free... On AMD the RMID limit is separate from the hardware counters that feed event count values into the RMID event counts. On AMD the two limits are significantly different. Babu shared [1] that the hardware of concern exposes 4096 RMID while it supports 32 (exposed to software) hardware counters when ABMC is enabled and 64 (not exposed to software) RMID when AMBC is disabled. resctrl could theoretically limit the number of monitor groups based on the number of RMID that could be accurately counted (with the caveat that resctrl cannot always determine this number) but based on the numbers supported by hardware that would severely limit the number of monitor groups users can create on this hardware. Users want to create a lot of monitor groups. Each monitor group contains a group of tasks that need to be monitored together. Creating a monitor group and assigning tasks to it is expensive since it involves the actual resctrl monitor group file hierarchy creation followed by interacting with these files to move the tasks into the monitoring group that internally involves changes to the various task_struct's. While resctrl could thus limit the number of monitor groups it would push the burden of monitoring many different task groups up to user space at significant cost. Instead, resctrl exposes a per-monitor group interface on AMD systems that support ABMC by enabling user space to more quickly switch which of the supported monitor groups are counted at a particular time. > >> The Intel/RDT spec does leave room for this to occur but a return of >> "Unavailable" when reading a monitoring event is not encountered on current >> Intel hardware. > > Why not? Because nothing's reaching that limit or? On Intel resctrl only considers the number of RMID. Hardware will return failure if there is an attempt to read an unsupported RMID that exceeds the limit it exposes, but resctrl does not do that. On Intel there is not a separate "hardware counter" limit to consider, only the number of RMID need to be considered and they are always counting. > >> The user can always expect a value when reading a monitoring event (on >> current hardware). > > How does that work when you exhaust the counters? Or it does switch between > them quickly so that there's always *something* that counts? On Intel the RMIDs are always counting. Please keep in mind that these RMID do not actually return raw counts that are verbatim passed on to user space but instead the returned value is "upscaled" using a hardware provided scaling factor to obtain the value that is returned to user space. This is done on AMD (with and without ABMC) and Intel. Reinette [1] https://lore.kernel.org/lkml/78996219-a8de-4dc3-90ee-db4c19e0d66a@amd.com/