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Mon, 23 Feb 2026 23:13:28 +0000 Message-ID: <0645bba3-6121-41d4-b627-323faf1089b7@intel.com> Date: Mon, 23 Feb 2026 15:13:24 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context switch handling To: "Moger, Babu" , "Luck, Tony" , "Ben Horgan" , "Moger, Babu" , "eranian@google.com" CC: Drew Fustini , "corbet@lwn.net" , "Dave.Martin@arm.com" , "james.morse@arm.com" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "peterz@infradead.org" , "juri.lelli@redhat.com" , "vincent.guittot@linaro.org" , "dietmar.eggemann@arm.com" , "rostedt@goodmis.org" , "bsegall@google.com" , "mgorman@suse.de" , "vschneid@redhat.com" , "akpm@linux-foundation.org" , "pawan.kumar.gupta@linux.intel.com" , "pmladek@suse.com" , "feng.tang@linux.alibaba.com" , "kees@kernel.org" , "arnd@arndb.de" , "fvdl@google.com" , "lirongqing@baidu.com" , "bhelgaas@google.com" , "seanjc@google.com" , "xin@zytor.com" , "Shukla, Manali" , "dapeng1.mi@linux.intel.com" , "chang.seok.bae@intel.com" , "Limonciello, Mario" , "naveen@kernel.org" , "elena.reshetova@intel.com" , "Lendacky, Thomas" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "peternewman@google.com" , "Shenoy, Gautham Ranjal" References: <679dcd01-05e5-476a-91dd-6d1d08637b3e@intel.com> <2b2d0168-307a-40c3-98fa-54902482e861@intel.com> <2416004a-5626-491d-819c-c470abbe0dd0@intel.com> <65c279fd-0e89-4a6a-b217-3184bd570e23@intel.com> <2ab556af-095b-422b-9396-f845c6fd0342@intel.com> <427e1550-94b1-4c58-828f-1f79e5c16847@amd.com> <37bc4dc5-c908-42cd-83c5-a0476fc9ec82@intel.com> <53be07ff-75c6-4bd6-a544-e28454b4f6b3@amd.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <53be07ff-75c6-4bd6-a544-e28454b4f6b3@amd.com> Content-Type: text/plain; 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The current status >>>>        quo for x86. >>>>     - global_assign_ctrl_inherit_mon >>>>        When active, CLOSID/control group can be assigned for *all* (hence, "global") >>>>        kernel work while all kernel work uses same RMID as user space. >>>>        Can only be supported on architecture where CLOSID and RMID are independent. >>>>        An arch may support this in hardware (RMID_EN=0?) or this can be done by resctrl during >>>>        context switch if the RMID is independent and the context switches cost is >>>>        considered "reasonable". >>>>        This supports use case https://lore.kernel.org/lkml/CABPqkBSq=cgn-am4qorA_VN0vsbpbfDePSi7gubicpROB1=djw@mail.gmail.com/ >>>>        for PLZA. >>>>     - global_assign_ctrl_assign_mon >>>>        When active the same resource group (CLOSID and RMID) can be assigned to >>>>        *all* kernel work. This could be any group, including the default group. >>>>        There may not be a use case for this but it could be useful as an intemediate >>>>        step of the mode that follow (more later). >>>>     - per_group_assign_ctrl_assign_mon >>>>        When active every resource group can be associated with another (or the same) >>>>        resource group. This association maps the resource group for user space work >>>>        to resource group for kernel work. This is similar to the "kernel_group" idea >>>>        presented in: >>>>        https://lore.kernel.org/lkml/aYyxAPdTFejzsE42@e134344.arm.com/ >>>>        This addresses use case https://lore.kernel.org/lkml/CABPqkBSq=cgn-am4qorA_VN0vsbpbfDePSi7gubicpROB1=djw@mail.gmail.com/ >>>>        for MPAM. >>> >>> All these new names and related information will go in global structure. >>> >>> Something like this.. >>> >>> Struct kern_mode { >>>         enum assoc_mode; >>>         struct rdtgroup *k_rdtgrp; >>>         ... >>> }; >>> >>> Not sure what other information will be required here. Will know once I stared working on it. >>> >>> This structure will be updated based on user echo's in "kernel_mode" and "kernel_mode_assignment". >> >> This looks to be a good start. I think keeping the rdtgroup association is good since >> it helps to easily display the name to user space while also providing access to the CLOSID >> and RMID that is assigned to the tasks. >> By placing them in their own structure instead of just globals it does make it easier to >> build on when some modes have different requirements wrt rdtgroup management. > > I am not clear on this comment. Can you please elaborate little bit? I believe what you propose should suffice for the initial support for PLZA. I do not see the PLZA enabling needing anything more complicated. As I understand for MPAM support there needs to be more state to track which privilege level tasks run at. So, when just considering how MPAM may build on this: The PARTID/PMG to run at when in kernel mode can be managed per group or per task. In either case I suspect that struct task_struct would need to include the kernel mode PARTID/PMG to support setting the correct kernel mode PARTID/PMG during context switching similar to what you coded up in this initial RFC. MPAM may choose to have struct task_struct be the only place to keep all state about which PARTID/PMG to run when in kernel mode but I suspect that may result in a lot of lock contention (user space could, for example, be able to lock up the entire system with a loop reading info/kernel_mode_assignment) so MPAM may choose to expand the struct kernel_mode introduced by PLZA to, (if kernel mode is managed per group) instead of one struct rdtgroup * contain a mapping of every resource group to the resource group that should be used for kernel mode work. This could be some staging/cache used between user space and all the task structures to help manage the state. I do not know what MPAM implementation may choose to do but as I see it your proposal provides a good foundation to build on since it establishes a global place, struct kernel_mode, where all such state can/should be stored instead of some unspecified group of global variables. Reinette